I have an DragonLady arf that is getting an SK 80 now but I am going to use a converted 20cc weedeater engine on it in the future. The weedie is well into the 40oz area. The SK 80 and the 20cc gas engine are fairly close in power and use the same prop.

My DragonLady has the holes in the rear of the fuselage to mount the rudder and elevator servos in to offset the weight of heavier engines.