Carl,
Maybe you can clean up the slot enough with a graver (maybe shop made from a Swiss file) to grind a screwdriver to fit the wider slot and trap it between a drill press vise and spindle to keep it from coming out of the slot (it may twist the head off though), after long term soaking with solvent as suggested above. You may need to turn the screwdriver with a vise grip, tap handle, or similar, because the spindle and vise are in the way.
Mike