Personally I would be hesitant to put a 5hp engine on the scott bonnar 45 frame, the frames have a tendancy of cracking as it is without the added weight and torque of a more powerful engine.
I have and a fellow forum member run a honda G100 engine which is 2.5hp 92.0 on the early ones) and they rip through thick grass like a hot knife through butter, recently I have bought another scott bonnar 45, which I have sourced a chonda (replica honda) g100 engine for, time will tell how that one goes.
At the end of the day the choice is yours, just check the measurements in the diagrams that are available for the new engines and go from there.
