Assuming you lost the hex hole... you can cut a groove with a hacksaw for a straight drive. Or grind flat edges on the screw heads for visegrip. If you lost the screw heads, you can drill and use an e-z out. I’ve had tough screws back out when reversing the drill (often because the screw head was gone and no longer forced against the surface.)
Good luck!
Forgot this reliable method... again, the screw head is virtually useless... use a steel chisel at about a 10 degree tangent to the edge of the screw head, and tap the chisel lightly with a hammer. You can first notch the screw head for a better grip. Definitely lube with the method.
No, the hex hole is just fine. I snapped off the business end of an Allen bit while trying to turn it with a socket wrench. A big enough chunk was sticking out, that I was able to yank it out with pliers.
I think my next step is obtaining a better grade of penetrating oil, and trying the tapping method that's been suggested.
Thanks for your help, Gene.