I saw a web sight that had a fellow show how to modify a Bi Turbo pipe to fit an original exhaust for an Avanti or a Garelli I cannot remember which. I'll look and see if I saved the web sight.
The sight showed how he cut the pipe from the BiTurbo and the original stock muffler. (That would mean that you would no longer have an original stock pipe and that you would really be screwed if you did it wrong)
You might wend an email to steves mopeds, I think that they installed a cylinder kit and new pipe for someone. Check on moped2.org