I have two Tower 75s and my son-in-law has one. We use Slim-Line pitts style mufflers for the OS 91 FX. It is very close, but you will have to use a small rat-tail file and file the holes slightly closer together on the muffler, then it will screw on fine. Also runs well with that muffler.

Now......I don't know how much good the Pitts style will do you if you are inverting the engine, but Slimline may make one of their Sport Scale (or whatever they call it) mufflers, which don't muffle much, but can point the stingers straight down on an inverted engine. If they make one for the OS 91 you should be in business.

Clair