I have a Sig J-3 cub with such an arrangement. i didn't build it, teh seller did. All he did was place the engine as far back as he could on the mount and adjust the throttle cable. It works fine and looks original. the front of the engine sticks out just a little, but no too much. it had a master Airscrew prop on it with an old, small rounded prop nut. i replaced the wheels with some off-brand,original looking ones that added to the scale appearance.
I once replaced the 4-stroke with a 40 2-stroek and did nothing to alter the CG or cowl.
but, I am back to the 4-stroke because it sounds and acts so much more scale-like.
[8D]