put a very thin and sharp blade (razor blade, Stanley blade) between the THICKNESS of the gasket in three places and cut it as far as you can go without other items like the frame getting in your way, then try the starter motor trick, if it fails, get some PLASTIC wedges as used for spacers in fitting house windows, and tap very gently, if it doesn't move something else is wrong, use very little force, it must come off the crankcase absolutely parralell, as their are two needle roller bearings in the outer case, and they wont go wonky without complaining.
CLEM