The 75 2 st will be fine BUT a 4 st will sound so much better. I was always really glad I put a 4 st in my 1/4 scale Stampe.

It would burble past in a very lifelike manner on slow passes.

On bipes with short noses it is common to need lead in the nose to get the CG right. Might as well put in a heavy engine and maybe run a large prop to keep the revs down and make it sound more realistic.