tamir_250,
Take a look at the Brillelli engines. I have the 26cc gasser on the CMP Giles 202 140 and it flys the plane great. It does not have the power for 3D but it flys aerobatics just fine. I am swinging an APC 17X6 prop at 8400 rpms. I also have the 40cc on a Somenzini YAK 54 73".
http://www.scottellingson.com/brillelli_003.htm