I've owned a .91FX and it will turn some rpm. I've run 12" props on it with an Ultra Thrust tuned muffler.

If your engine knocked and quit very hot, you are running it way too lean. An engine will lean out in the air due to ram air in the intake and g forces, so you always need to set it slightly rich on the ground. I like mine to leave a little smoke trail in the air at the first of the flight. After 3 or 4 minutes, the engine leans out enough so the smoke trail is gone.

I recommend an OS #8 plug and some Omega 10% castor blend fuel.

If you really want to go, try a Jett tuned muffler and a 12-6 APC. Just don't run it over lean.