A Saito 100 with an APC 15x6 is a perfect match for spirited sport flying. Very unlimited- it would give you all the vert that you could want. I flew this combo for a while.

If you have a little more money to burn a YS 110 would drop right in. It would be an insane amount of power (the YS 110 is more in the Saito 150 class) but it would be lots of fun. The 110 would make the plane accelerate faster on a vertical than the Saito 100 would fly it level.

Both of these engines are more than is really needed to fly the plane, but if you are going to buy a new 4-stroke engine for it then you might as well do it right!

Walt