"...Looks like someone let it corrode badly, and then covered it up with Gunkote!"

With that as a history, before you do any adjustments take it all apart. Clean and inspect everything.
Check for damaged or altered parts and surfaces. Sounds like it's been apart already in the past. Maybe some 'enhancements' have been performed,,maybe not. Once it's apart and clean you can tell.


Many times a good cleaning will show you better what needs to be done to repair a problem and sometimes clears the problem all together.