The SPE 26 would be a great motor friend has one and it is light and seems very powerfull. Looks about the same size as a G20 but way more power. Here is another one I came upon not sure it would work havn't measured distance from firewall to prop hub in a while but with rear carb it might fit more applications and is cheap. Need to look around first and see if anybody is using one and if they run good. If anybody knows anything about them let me know. $235 with wrap around muffler and standoffs is a great price.

http://www.lsproengines.com/lspro_004.htm

Randy