Get a good 2 stroke engine...
Why fight weight putting a 4 stroker on it when u can have a much lighter 2 stroke with plenty of power...?
I think a good YS .80 heli or OS.91 FX will do fine.
Whenever u stay under 6lbs.
Mine was 6.1 and did great Walls, harriers, elevators, killer flat inverted spins, but I still agree that may not be a very forgiver 3D trainer.

Remeber that slightly tail heavy planes are easier to 3d than nose heavy ones, but take easy the CG relocation on this Cap.
Greetings