If I'm not mistaken, you can use a 4 stroke in SPA up to .91 sized, BUT it can't be supercharged etc. I'm not exactly sure about the standard YS fuel system.

The only thing that keeps me away from SPA is you can't run tuned pipes or retracts. the guys with the .91 4 strokes are running away with it. Let me fly my piped UFO and I'll fly it =)

-Mike