Use a very thin dremel cutoff wheel or a small file to cut slots in the exposed end of the screws, then use a screwdriver to remove them like any other screw.
It would probably be a good idea to heat the case in the oven or with a torch before you start trying to unscrew them. The heat will help in two ways; it should help loosen up the loctite, and it will also expand the tolerances between the screw and the threaded hole.
Make sure you plug the exhaust and carburator while you are grinding (or filing) on the screws, and don't burn yourself!!