Repacking a Muzzy
There is a kit available from Muzzys for repacking the insulation material in your muffler. It's about $25. You'll need a pop rivet tool (about $10) and rivets (1/8" diameter, ~3/8" length) to put it all back together. The end caps are stainless steel and are reused.
The repair kit consists of the perforated inner tube wrapped in fiberglass. If you could get the right fiberglass material from a muffler shop or auto parts store, there is no reason you couldn't just replace this bit. Look for material for automotive glass pack mufflers.
What you'll most likely see when you open up the can is that what little fiberglass is left in it has settled to the bottom half, so there is no sound absorbing material at all in the top half. (And you were wondering why it was so loud.) With the new packing, the bike should be quite a bit quieter than it was. Still loud, but tolerable.
To install this, you have to drill out the rivets and remove the end cap (sounds simple, but it's awkward and takes a bit of 'delicate force'. One member recommended seeing if Muzzy would do this for you.)
Only one end cap needs to come off, and the engine side cap is easier to get off, since the mounting hardware provides something to pull on.
1. Remove the canister from the header, and use an 1/8" drill bit to remove the old rivets from the endcap closest to the header (this endcap is out of sight if you damage it, and has a little more to grab on to when you pull it).
2. Carefully pull the endcap off. This will likely take a decent amount of force; try to use equal force around the endcap so as not to damage it. With the endcap off, you can look inside the canister and see what's left.
3. Now, pull the baffle out, along with the remaining packing. In the picture below, about 1/2 of the packing was gone after 10,000 miles. When repacked the first time (the packing having been done by Muzzy at the factory), there was only 1/3 of the packing left after 8000 miles.
4. Clean off the baffle, and lay out your new packing materials. You'll need one or two packages of Silent Sport or similar packing material (available at any local dirtbike shop), a roll of steel wool, and some picture hanging wire or similar wire.
5. First, wrap the baffle in steel wool blankets, unwrapped from the roll. This will keep the fiberglass packing from breaking up as quickly from heat cycling. It will also prevent the packing from leaving the pipe very soon after it begins to break up.
6. Then wrap the pipe in one or two sheets of fiberglass packing, depending on how much sound deadening you want. Use the picture wire to tie down the packing material, so it stays in place while you pack the canister with it, and to further reduce packing breakup once in use.
7. Slowly insert the new baffle core into the canister, making sure the packing doesn't bind as it is inserted. Wiggle the core around in the pipe once fully inserted to mate the core with the far endcap. Replace the front endcap, lining up the clamp with the downturn in the pipe on the other endcap. It may help to tap it into place with a rubber mallet, but do not use much force. Use new 1/8" (3 mm) short aluminum or stainless steel rivets and a pop-rivet tool ($10) to secure it. The aluminum rivets will be easier to drill out next time, but they may also vibrate loose between repackings.
If the pipe is not quiet enough for you, you can order a new baffle (Quiet Core) from Muzzy that has a smaller core. This will reduce pipe performance slightly, however, and may require a rejet.