Any weedie conversion isn't going to have rediculous power. They are fine for a 4* type plane.

How much vertical? It depends on your altitude and the weight of the plane. I've read on here that a 4*120 with a Ryobi 31cc will go outta site, but I think that guy was at sea level.

25cc will fly it fine. You'll be able to pull back and do a nice round loop and have enough vertical for a nice hammerhead. I don't think a 4*120 will hover with a 25cc engine. But, it's not a plane that your supposed to be hovering anyway. It's a nice sport plane and it will do just fine with a 25cc conversion.