This web page lists some tips for hot-rodding your Musicmaster. Until at least when the Duo-Sonic was discontinued (ca. 1967), both the Musicmaster and the Duo-Sonic shared the same bodies, with both models routed for a bridge pickup. (The Bronco-like mid-late '70s MMs were routed for only one pickup; I'm not sure about the early '70s "Music Master" guitars.) Thus, to add a single-coil-sized bridge pickup to a Musicmaster or Musicmaster II, all one has to do is rout the pickguard for the new pickup and the switch(es). Both of my Musicmasters have had this mod done to them (my red late '50s MM came that way, while I modified my white '65 MM2 myself). Of course, you can also add a full-sized humbucker, but that would require extensive body routing and is beyond the scope of this web page.
Routing the pickguard
Determining where to rout the pickguard is the most difficult part. If you have a vintage Duo-Sonic (or Mustang) pickguard for comparison, routing a Musicmaster guard will probably be easy. Remember to allow for the warping and shrinkage of the plastic used in pickguards thru at least '65, if applicable. I didn't have anything to go on, so I had to use a protracter, a pair of calipers, a straight edge, and a lot of guesstimation with my '65 MM2. I found a couple of Mustang 'guard images on eBay, one from the '60s and one from '78: if you print them out at the proper scale, you should be able to use these images as templates (compare the distance from various points, like corresponding screw holes, with your existing 'guard to make sure the scale is correct). For pre-'64 guitars, feel free to use this scan of the back of my '56 Duo-Sonic's original pickguard (122 k; scanned at 150 dpi) for the rout locations. If you have an old Duo-Sonic (or Mustang) guard to spare for your Musicmaster, that's even better. I think old-style Mustang 'guards are still available from Smart Parts, and you can always go the custom route (such as Warmoth, Jeannie Pickguards, WD Music Products, or Pickguard Heaven).
Because I didn't have anything to go on, I drew a diagram of where the rout needed to be on the bottom of the guard in pencil. First I screwed the (somewhat warped) pickguard upside down into a large, flat 1/2" thick piece of wood. The neck pickup is angled at about 19 degrees from the vertical (or about 71 degrees relative to the strings). The length and width of the rout was determined from the neck pickup rout. The routing itself was done with the 'guard/board assembly clamped under a drill press using a drill bit that approximately matched the radius of the pickup rout corners (unfortunately, I forgot the exact size). The difficult part was determining exactly where the rout should end and begin, to properly align the pickup rout in the 'guard with the existing rout in the body. I made a centerline on the 'guard from the truss rod adjustment recess at the neck end and the pickguard mounting screw hole near the bridge end, and then made parallel lines that intersected the neck pickups' screw mounting holes and the tip of the neck pickguard rout. Once this was done, I determined how much clearance the neck pickup had in its rout, and estimated about where the bridge pickup rout must lie. Then I drew a line parallel to the new rout, angled approximately 71 degrees from the strings. If this sounds like an imprecise process, you are correct; since the 'guard had warped over the years, it was even less precise. The final result was not perfectly aligned to the neck rout, but it was fairly close. I used a rounded-profile ("rat tail"?) and a flat file to smooth out the edges and for final shaping.
The type of bridge pickup you add is up to you. If you want a near-exact copy of vintage MM/DS pickups, Seymour Duncan offers Antiquity Duo-Sonic pickups, listing for $99 each; ask for parts #1034-01 (neck) and 1034-02 (bridge). Both pickups have had the polepieces ground flat, and they are wired such that the middle position (both pickups on) is hum-canceling. Otherwise, I think they're identical to their Vintage Staggered pickups (a great sounding pickup from my experience). They offer many other models of Strat replacement pickups that will work with MM/DS guitars, as do other pickup manufacturers. But whatever pickup brand or type you choose, remember that the height of a pickup is an important factor. Strats can have deeper pickup routs than any MM/DS because the body has to be thicker for the vibrato system. If a pickup has two huge bar magnets glued to the bottom, once installed there may be little room for height adjustment or for wires to pass beneath it. Pickup routs in my '50s MM/DS and my '65 MM2 are 5/8" and 3/4" deep, respectively. You won't have as many problems in the bridge position, because the pickup can be closer to the strings, but it can be a big issue (no pun intended) for replacement neck pickups. In an effort to gain enough clearance to fit a Duncan Classic Stack in my '65 MM2, I had to scrape away the lacquer in the neck pickup rout, and it was still too close to the strings (eventually, I gave up and replaced the Classic Stack with a Duncan Vintage Staggered single-coil).
I've read several people state that the bridge pickup on a stock Duo-Sonic (especially the older ones, which weren't angled) sounds weak and tinny relative to the neck pickup. The shorter scale length and corresponding lower string tension in today's light string gauges means less string vibration, especially near the bridge, and lower output. This is endemic to all guitars (most modern replacement bridge pickups have higher output than corresponding neck models), but the effect is more pronounced on guitars with a shorter scale length. To rectify this, consider adding a higher-output single-coil or humbucker pickup. Personally, I've been pleased with the Seymour Duncan Hot Rails for Strat (single-coil-sized humbucker). It has a pronounced midrange tone (if you hate mids, stay away from this pickup), and is very high output (16.9 k Ohms; expect some dirt in your sound even thru a clean amp). If you play clean most of the time, you might consider a Cool Rails for Strat pickup (~10.3 k Ohms), or another one that's just a little bit hotter than average. I've been told the Duncan Li'l '59 also works well.
The last issue about replacement Strat pickups adapted for the MM/DS is the direction of mounting. If you want to avoid routing the body for a replacement bridge pickup, note that the shape of the bridge pickup rout on a Duo-Sonic is a mirror image of the neck pickup rout, instead of an exact (angled) copy like a Strat. Because the Hot Rails is a rail design, it can be installed "backwards" (mounted such that the normally neck-facing side faces the bridge, and vice versa) without any problems. If the pickup you're thinking about has staggered pole pieces, be aware that both Fender and Seymour Duncan offer "left-handed" pickups for some models, that can be installed backwards with no problems.
SwitchesFinally, you have to determine where to put the pickup selector switch(es). For pre-'64 Duo-Sonics, I do not know where to obtain a replica of the original toggle switch (although I've been told that Danelectro makes a reasonable copy; see parts sources for more info). In my '65 Musicmaster II project, I installed a Gibson-style 3-way toggle switch in the treble-side body horn, as seen on earlier Duo-Sonics, rather than adding a pair of 3-way slider switches. This required significant routing to the body of the guitar (done with a drill press! see below), and I lost one of the 4 pickup combinations available to the Duo-Sonic II and the Mustang (both pickups, out-of-phase). However, it put the switch further away from the strings, so that they're less likely to get in my way when playing, and I really didn't value the thin, nasal tone from the lost pickup setting anyway. If you use a drill press or have to clamp the body for routing, ALWAYS use wood clamps, NOT metal C-clamps (even with padding).
When installing anything inside a pickguard, an important consideration is the physical volume the switch will occupy when installed; the resulting body rout must be covered by the guard. I used an AllParts "Short Straight Toggle Switch" (part# EP 066), and centered it 1" from the tip of the pickguard's treble-side horn. This switch requires a rout at least 15/16" deep. I made a 1 1/8" deep oval-shaped rout, with a 3/4" deep by 3/8" wide wiring channel to the neck pickup rout. Subsequent wiring of the switch would have been much easier if I routed a channel from the neck pickup to the bridge pickup on the treble side of the body (and completely ignored the slider switch cavity), but I felt this would have decreased the resonance of the guitar body. Note that straight toggle switches allow movement up-and-down, rather than left-to-right like stock Duo-Sonic toggle switches. I don't know how well they would fit in old Duo-Sonic bodies, but also available are right-angle toggle switches (as used on old Gibson SGs and the current Fender Cyclone), which have a similar design but allow left-to-right switch movement.
A wiring diagram for the '56-'64 Duo-Sonic was online at Seymour Duncan's web page, but it must have gotten lost when they redesigned their website. In the meantime, a copy of their wiring diagram is online here. When you actually install the toggle switch, be sure you're gentle with it! You don't want to risk messing up its delicate leaf springs. If you like the post-'64 slider switches, I think the replacements available from StewMac (part #1232) come with diagrams, and a stock Duo-Sonic II/Mustang wiring diagram is now online. Check the parts page for more parts options/sources.