I don't know if they make what I'm about to mention small enough but here goes, step 1 drill a small hole into the centre of the screw should be able to get one small enough, Step 2 use an easy out in the hole you have just drilled in the screw(not sure if you can get one small enough) this should bite into the hole in the screw and grip it well enough to unscrew it. If the screw is proud and not recessed then I would carefully cut a line going straight across the screw with a small hack saw creating a new slot for a normal screwdriver but I'm guessing the screws are recessed.