You can try Carr Precision and/or www.lambertsrc.com. I don't know of any others at the moment that are making mufflers/parts for conversion engines. Actually due to a health problem, I don't think Lambert's is making them anymore, though he might have one already in stock for you so it is worth checking.

I usually just cut the stock muffler apart and gut it. If it is large, I cut it down to size, weld a piece of thin steel over the opening, and weld outlet tube(s) into that. They seem to be lighter actually than purpose made aluminum mufflers.

I had a bad computer crash and haven't re-loaded all my pics yet, so sorry I don't have some good ones to post. Here though, is one of the muffler I made for my big Fokker Dr1. This one was more involved as I needed it to be narrow. I took a piece of automotive muffler tubing, squashed it flatter, welded endplates on it, (one of them with two outlet tubes), welded in anti squash tubes in the mounting holes, and cut an opening for the exhaust port. Admittedly this was pretty involved; most of the ones I make are pretty simple to do. Just don't try to use regular solder nor even the Sta-Brite Silver Solder they sell at hardware stores. It will melt instantly. You need to use either real silver solder or braze, (very expensive), steel, or bronze braze.

AV8TOR