A little heat with a propane torch will go a long way. Doesn't take a lot. Heat the screw head, squirt on penetrating oil and heat again and repeat several times. If that doesn't get it loose, chuck a proper size screw driver tip in a drill press, next put the piece solidly in the drill press and with one hand pressing the screw firmly into the screw slot using the drill press, turn the drill press chuck with your hand. Don't try to power the screw off, all you want is to be able to press the screw driver tip solidly into the screw slot. Be aware that you can easily put enough torque on the screw head to split it in two so be careful.
Kroil can be hard to find in the stores, their address is
Kano Laboratories
1000 E. Thompson Ln.
Nashville TN 37211
(615)833-4101
I hope this helps.
Steve
