The stumbling when you roll inverted is probably due to the tank being too low when it is upright (causing tank to be too high when inverted). To help insure you are tuned correctly and not too rich on the high speed needle. Another thing to try would be to move the tank so the spray bar of the carburetor is about lined up with the center of the tank (or real close), this may be difficult to do on that plane. Another thing that may help if the above does not help is to get a hotter glow plug.