All good advise posted here. We use to race these planes with that very engine. After trying every prop under the sun, we had best results with either an APC 10x6 or APC 10x7. Anything bigger, did not go any faster, and the smaller 9" props just didn't have enough thrust.

If you haven't removed your muffler Baffle, you can do that for a small boost. A cheap way of increasing speed would be to purchase a Tower Muffler for your engine. This will increase your RPM. The Ultrathrust muffler would work even better.

The only problems with these mufflers, is that the installation will be tough unless you run the engine upright, or inverted. A standard side mount won't let the muffler clear the bottom of the fuze.