I had a .60FP in a four star 60, and it was a little weak for the airplane. It's okay if your going to just buzz around the field and have fun, but if you want to do any aerobatics I feel that you will be dissappointed. A 60 size ballbearing engine is the minimum 2 stroke that I would put in it.
I currently have an OS .91 four stroke in it, which seems to be about perfect for this plane, but still slightly shy in the guts department. I am thinking about a Saito 100, or a YS 91 for a power upgrade. Big four strokes seem to fly this airplane better along with the cool sound!