Best I remember, mine were no joy to remove either. They used some type of sealant on them that had a pretty good locktite effect.

If the slots are pretty much gone, You might have to drill the heads off of the screws using a drill. I usually start with a small pilot hole in the middle of the screw and move up through bigger bits. Eventually, you will sperate the head from the bolt. Then you should be able to remove the factory mount and grasp the remaining screw with some vise-grips or such and back it out.

Be sure not to drill all the way through the screw as it is open to the cranckcase area and that could introduce metal shavings into the cranckcase.

Anybody else got any ideas?