Sure. You may lose some flat out speed and acceleration. You'll also probably have to add some nose weight (further slogging down the performance) if the model is designed for a 4-stroke. I have an OS-70II in my Contender 60 and it still scoots along.

As far as prop? Sometimes a personal preference. 11 x 7 would work on the .60. A 13 x 6 would probably give a more scale performance for a Fokker.