One thing you need to look at besides horse power is torque. Most engines don't list it, but you can get an idea of how they compare by looking at what props are recommended for each engine. The .46 can use an 11x8 or a 12x7, the .70 can spin an 11x9 or a 12x8, and the 90 can take a 14x7.

Personally, I think the 70 4s would be just fine on this plane. There's no need to overpower a plane that's meant to be a trainer.

Kerry