I've got the rear camshaft OS FS-90 as well...!
The PIPE Here again - for the Enya mills, W. Bill Jensen IS your man, and he is sort of "in my backyard", being in New England - Bill is in CT, and I'm in MA !!
The rear camshaft OS FS-90 has an M11 x 0.75mm exhaust port thread, and can DEFINITELY take ANY muffler hardware meant for the OS FS-91 Surpass, as I've checked that combo myself
quite some time ago.
The OS FS-20 MIGHT be able to use the same exhaust hardware as the FS-26 and FS-30, with the FS-30 definitely having an M9 x 0.75mm thread spec, exactly the same as the 21st century-produced Saito FA-30S enclosed drivetrain four-stroke and the currently produced FA-40a also use, and which I both have (on the two small Saito mills).
Hope these tips also help you out...