I've had that problem. (Per Mike, fill with water.)
Drill out the screw. Tap bigger, open sender flange hole, use next larger screw.
If the area around the tank hole is thick enough, abandon the screw, tap a new hole next to it and drill new clearance hole/slot in sender flange. It's got a gasket and no pressure and is on top. So it's just for splash out, if vented cap is good. No big seal deal.
There are some welding ideas, but not for me.
A new tank is $150. Wonder if that is less stress?
