You might want to take a look at this exhaust system:

https://www.1977mopeds.com/cgi-bin/c...=action&key=97

It's a 2-piece bi-turbo, an aftermarket expansion chamber muffler that a lot of mopeders use. Because the muffler and header are separate pieces, it's easy to modify to fit a lot of different bikes. These kinds of exhausts usually give you about a 3-5mph boost on a moped, probably similar results on a motorized bicycle.