I typically fabricate bridge adapters to adapt mufflers for some of my engines. I have made a number of the adapters for my purposes already.
You don't need a lot of fancy tools to make these either. A set of files, hacksaw, and a drill (although a drill press is better).

here is where I made a bridge adapter so I could use a Performance specialties (aka Nelson) tuned muffler on a Fox Eagle IV .60 engine. it is a real screamer and powerhouse like this as the stock muffler is very restrictive. A Rossi .61 doesn't run any better than this setup with the same muffler and prop even. the muffler was made for a OS .61SF type of engine. Using two pieces of metal allows you to drill the holes for the different bolt spacings to fit properly. In some cases one only needs once piece of metal if the holes don't interfere with each other.


here is my Rossi .61 with a Bridge adapter for the same muffler.


here is where I made an adapter to fit a old K&B muffler onto a old early version Fox .45 engine, where the bolt holes are about 2mm closer together than on the models that followed it.