[Austin W Hogan]

It might be possible to grip that remaining piece of screw head with " needle nose " vise grips which are probably available at a good hardware store. Soak the visible piece of the threads with oil overnight before trying it. Penetrating oil may remove old color if it gets on the surface.
E Z Out makes reverse direction drills; if you have a reversible drill this may work; the small E Z outs are also available at good hardware stores. I have removed hammer screws and a frame screw from lifters this way. The frame screw had lost all hardness and crumbled both when I tried a screw driver, and when I tried to turn the E Z out; I was able to pick the pieces from the hole and the threads remained intact. You may have to clamp the hammer as well as the lock when you drill, to keep the lock axle in place.

Best, Austin