I would take the dimensions of the stock silencer (and mounting brackets) and see how they stack up to the offerings from FMF or Pro Circus. You may find something close that with a little engineering would fit your requirements.
Quite regrettably but, you are left with trying to match up the od of your pipe outlet to a universal muffler.And happy engineering to get it to stay on. My 370/400 has a pos 125 pit bike muffler, and a universal clamp, cheep! I do not know, but give Vintage Suzuki a yell, he may know a guy? My favorite old trail bike, keep us posted! Vintage Bob