It depends on what you want to do. I've used the same APC prop that proptop mentioned with good success having only the Tower muffler. That gave me 1000 more RPM. I think going to a hotter glow plug did more good than the new muffler.

This engine will swing a 12" prop just fine. From there, you can prop it for speed or prop it for vertical like proptop and I did.

The first thing to do is put an A3 plug in the engine.