I think that the single cylinder will have more HP for the same displacement twin. I have never owned a single but I guess I would recommend something large that will rev as high as possible.

I think there are some 4 strokes out there made for cars so you wouldn't need a fan. Oops, I just looked at OS engines and the 4 stroke car engine is tiny.

Make sure you gear it up. I like the twin to keep the COG down lower, maybe you could mount the single on it's side ( don't have the glow plug pointing down).

I have an OS and Saito and like them both. Go to the engine forum to look for more info. Oh ya, a built in fuel pump and/or supercharger would be very cool.