http://www.horizonhobby.com/products...90-arf-SEA5050

[h=3]Needed To Complete[/h].91–1.25 2- or 4-Stroke Glow Engine
Glow Plug (Engine Specific)
Silicone Fuel Line
6-Channel Radio System with 6 Digital Servos
Propeller
2.75" (70mm) Spinner

Probably the largest 4 stroke you can buy to put in it, you don't want to go too large, the weight penalty will offset the power gained and make it somewhat difficult to balance.

http://www.horizonhobby.com/products...EA5050#t4-KITS
This is the recommended completion kit, you don't have to buy their servos, but look at the specs and try to find the equivalent from somewhere that suits you. The recommended 125 is an animal, and once broken in, should haul you around with no problem. Make sure to completely epoxy the firewall to avoid failure in that area.