Hmmmm, do you want aftermarket high performance or aftermarket OEM replacement? Musket in NZ would do a replacemt header pipe. i will have a look, because i think TK pipes (australian brand) may do something aftermarket for the 185
yes just checked and TK pipes do a aftermarket performance ADR sport muffler for 79 onwards XL 185's. slips onto standard header and would eliminate any mid section muffler box if there is one. DEP pipes in UK may also do a muffler too, not sure though. and i am pretty sure Musket do a replacement header.
also try powroll in USA too.
also Supertrap, White brothers and FMF do universal four stroke mufflers you could use.