Parts specific to the old O.S. FS-90 are no longer available. We'd not have them, nor would O.S. The engine's been out of production for too long.

We do have the exhaust pipes for the FT-160, which will fit. They do not have a muffler, though. The part is:

OSMG5756 46169000 EXH PIPE ASSY T120160 $17.99 (Includes the retainer nut)

These parts can be ordered through any hobby retailer who sells O.S. Max engines and accessories.