If I were in your position, I'd be working on providing better cooling and dialing in my O.S. Max .52 4-stroke. I think you have just about the perfect engine on your Spacewalker II, you just need to get it running correctly. If you can get a 12x6 propeller turning around 9500 rpms without your engine overheating, it should look, fly, and sound perfect.

You have the perfect engine for your airframe, now you just need to get it to run right.