a guy at my field runs a 61fx in his, flys great and pretty fast, but 12" prop is about it with standard wheels

You can use the 4 stroke and it will fit, but you will need to run a 12x10 prop or similar. This will work out great for you because you are looking for speed not trust with this plane.

Personaly I think this is one of the few planes that sounds "scale" with a 2 stroke, If I were you I would run a tower .75 and put the 4 stroke in something else...................ever heard a jet with a 4 stroke?