RE: What sport/speed engine do you want to try next?
I'm just about to get my feet wet in this game. I've got two projects waiting for me to build.
The first is a Shrike 10. I'm going to use a K&B 3.5 cc rear exhaust with tuned pipe. I've never run this engine in aero format, but if it works like my outboards, it should be good. Since this plane is small and simple, I hope to begin soon.
Next, while not particularly known for speed, is the K&B .61 Twister that I just ordered. At $80 I figured it was worth a try. Going to put it in the GP F-15 I just got off that "other" auction site.