RE: Gas powered trainer
When flying with the gang at Brazoria Field in Texas I flew a 25cc powered Sig Four Star in the 120 size; it had about 14 lbs thrust and weighed about 14 lbs. That gave enough vertical for basic aerobatics and the handling characteristics of the plane were pretty good. Ever tried to do a roll with an eight foot Telemaster? Yikes!! The Four Star is far better than any of the high wing offerings when it comes to basic aerobatics.
I am currently designing and building an 80 inch version of my design, "Duette". With the 26 cc SPE I am expecting it to do basic aerobatics very well and to serve as a basic flight school trainer.
When teaching new flyers, I have found that the same flying characteristis of a good aerobatic airplane are a help to students in learning to fly. The typical trainer is full of problems like adverse yaw, control coupling, and too much stability. A low wing like the Four Star is probably a better choice for basic training than the typical high wing trainer. Art Cloutier