Ed - you can't compare the same size displacement 2-strokes with their 4-stroke counterparts. a 91 2-stroke is significantly more powerful than a 91 4-stroke. but then you already knew that :-)

as for the 13x6, it's true that 4 strokes will spin stonking props. but then are RCV SP engines the brute of them all spinning 13x13 props? try that on your saito! different props work differently for different engines and different applications. but all elses equal, 2 strokes are just more powerful for the same displacement.

anyway, i agree with ed in saying that the 52 4-stroke will definitely be enough for the trainer. so if you wants to go fourstroke, so be it. but i'd still recommend the 2-stroke.