Stick a big chunk of modeling clay, or a wax toilet bowl ring in the
opening on the outside of the hull before you start. David's ideas are
good. I find the best thing I've ever found to free stuck metal parts,
screws, bolts, etc. is PB Blaster. You should be able to find it at an
auto parts place. Spray it wherever you can and let it soak. Tap
lightly with a hammer on the body, stem, etc. then see if that frees it
up. If not, soak some more. Once you free it up, lube with some
plumber's silicone grease. Don't forget to remove that outside plug
before running again!