If it's a joint I'd use Ultra High Temp Copper Silicone on the male end of the pipes and fit them both together, if it's a crack or broken weld then a tin can soft patch and hose clamps will work as good as anything. If there's a noticable gap you could use muffler tape, the kind you soak first then wrap around the male end to almost the i.d of the female end, slide them in (ahem) tighten them up and run the bike to get the tape to harden.
One final thought if all else fails try to find some graphite re-inforced shoe-string valve packing and stuff that in there. If there's a shipyard or repair facility or plumming shop near by they should have some
The things are bricks: you can chip 'em but they still hold up. 21 years later (86 model) and gosh, I had to sweat out how to get the surface rust off the little bracket that holds the muffler can on...it's almost boring.
You don't need any sealant. You need a chunk of 00 steel wool. After the carbon and crap around the exhaust joints starts to get annoying , rub the steel wool on it till it's gone. There's a service bulletin from HD (8585-77FX) for the 77 Shovehead on that with more detail. If the noise is bothering you, get some earplugs. Your bike will run much better with them in. (HD bulletin 9934-77FX)
Wow, this has been a big help. Spider webs? Cow dung? Why didn't I think of that?
But seriously folks, here's the deal. At the collector / muffler joint, I did successfully use a combination of fiberglass and 2-part Epoxsteel, similar to JB Weld. No, I can't find JB weld itself. But I can't do the same at the rear header pipe, because there just physically isn't enough space to get in there and do the work. There's a big frame member in the way, clearance is tight.
I've now got some hi-temp RTV silicone gasket material, rated to 685 F. I've also got some muffler tape today. The whole system has been taken apart. Joints will be cleaned and prepped, and wrapped with tape as a "shim" material. The outer surface of the tape will be covered in a thin layer of RTV (to protect against exhaust gasses), slipped back together, and tightened with hose clamps. Think it should work.
No, I can't weld it. No clearance, and even if there was, it has to be removable in the future should there be any engine work required later.
As they say in China, "Jia You!" (literally, "Give 'er gas!")
On my first bike ('71 CL100 at age 11), I used a doubled-over Schlitz can, some pop-rivets, and a bit of clear silicone-sealant to fix the holes. If it's still running and in one piece (F'N THIEVES!), I'd say the rest of the muffler has probably rotted-away leaving the Schlitz can behind...............