If those are from a gottlieb you may want to just heat them like uncivil_engineer suggests. They are probably 6-32 which my Lowes didn't have and the ones I found at ace had a larger diameter base and wouldn't fit in the countersunk hole on the bottom of the play field. Just hold the stub of the screw with some pliers and heat the nut to break down the loctite. A soldering iron will work but a propane torch will get it done in a few seconds. The smell of the loctite burning off isn't the greatest so you may want to do it outside.