RE: GP Ultimate Bipe and engine size?
Either one of those engines will fly the ultimate. The magnum will be great. I have one of these and I fly it with an OS 46 FX and it has plenty of power to do everything but hover. It would probably hover but I am not a skilled enough flyer to try it. My bipe will do everything that I am capable of with the 46 on it so the 4 stroke should be fine.
Rick