You can also go with an O.S. .32 as one more option as you're going to find out that the .21 will need a new motor mount fabricated. That engine is still classified as a sport engine but better than the stock Dynamite
As far as the poor handling at speed, that's an easy fix. The problem lies in the Pro Boat placed the trim tabs too far out so when the boat is up on plane, the trim tabs aren't or barely touching the water and become ineffective. Move them closer to the strut.