The stopper on my tub jams up and won't allow me to stop the drain. As part of the initial stages of repair, I took off the cover plate and pulled the stopper assembly out, but the threaded ends of the cover plate bolts rusted into where they were screwed into on the overflow drain. Any thoughts on how I can fix this without calling a plumber? I have thought about just using a rubber stopper to plug the drain whenever needed, but the overflow needs to be covered or plated. FWIW, my house was built in 1968 and has cast iron drains.