Two muffler would work....

1) the 90 Torque muffler - aka JS-120 size jett-stream muffler. It is just a little longer than the regular 60-90 size jett-stream.

That is the one Jett offers for the new OS120AX, and what was used on the Jett 100L and 120L engines.
Lower tuning band ----- 10K to 12K rpm is where you want to prop for at full throttle.
If your desire is big props and the lower rpm band..... this would be the choice.

My notes show that the SF bolt spacing is the same as the OS91, so it would be an off-the-shelf item - no adapter.

2) the turbo-jett 20cc size. This is the in-cowl muffler. Again, same one used on the Jett 100L and OS120 engines. Same range, same sorta boost, different packaging... not always suited for the front end of a traditional pattern ship.

Just of comparitive note, the OS 91FX with the "regular" jett 60-90 size muffler turns a 12x8 around 12,500-13,000 ground peak rpm - really jumps on it in the air. About the same with the OS61fx and an 11x8.