I have a GP Spacewalker with a Saito 100 and although the engine is very strong the plane is very under powered. I realize this is no pattern plane but I would like to be able to do a 1/2 roll in the vertical position, but with this engine it is falling out of the sky before 1/2 roll is completed. If I really liked the plane ( and I don't ) I would put a 120 or maybe even a 150 in it.

A 120 twin, too much power !! I don't believe it.