You really don't have a lot of choices with rusted, damaged, screws. First option is to drill them out with a bit made of harder material than the screw, carbide, diamond, whatever. Second option is a small grinding tool such as found on hobby tools like the Dremel line, also some hardware stores have an interesting array of grinding (and cutting) bits.. This all takes a steady hand and patience. A third (least desirable) option is a small sharp chisel, (you may have to grind one down), It's often possible to chisel a slot in a screw head to remove it with a standard blade screwdriver but don't go crazy, in this situation you'll be pounding into plastics, use only is if all else fails. The material can't be all that hard because it's sure easy to mess up the head with the called for screw driver. Haven't been there yet so don't know if you can get behind the assemblies, doubt it, but if so, small vise grips can be used to back the screws out from the inside.