A better description of the screw would help. Is the threaded part likely to be long ? if so penetrating oil or WD40 is not going to help. Placing a screwdiver in the slot and tapping it with a hammer could help, but is the casing of the generator likely to be broken by the impact? A nail-punch or something similar can be placed at the end of the slot, held obliquely and tapped in an anti-clockwise direction. If all else fails, a hole can be drilled into the screw and tapered tool with a left-handed thread can be screwed into the screw. I have a set, but cany remember what they are called, perhaps 'Screw-Outs'. Any mechanic should know of them.