Getting stripped screws uot is like the other poster recommended, use a dremil and cut a slot in the top that you can then remove it with a slotted screwdriver. As for your scar not firing, mine had a similar issue awhile ago and to fix it you DO need to fully disassemble the gearbox. In mine the electrical contact that engages when you pull the trigger didn't have enough pressure applied when the trigger was fully forward resulting in a half cycle on the piston and then it would quit. Taking the gearbox apart with the piston half cocked resulted in a broken tappet plate so watch for that. To fix the electrical issue all that was required was to bend the two contact plates together that the trigger pushes into.
Item "H" is the electrical block you are looking for. Hope this helps.