If you flip to page 40 of your November 1989 dealer issue of Moped Mart, you'll find this exhaust on the bottom left.
This is the 24mm clamp version. Overall length of 20 inches. From mount to tip of baffle is 11.5 inches.
"MG-442C, Tuned performance black muffler for use on Garelli model mopeds or any other units with a L. H. mounted exhaust system"
Part Number: MG-442C