Knowing pretty much nothing about bike mechanics - I'd make the bike comfortably ridable. So replace the stem, seat, new tape. Also you want to make certain the brakes actually work. Stopping is good. If the gears are in reasonably good shape - not missing or bent, then I'd stick with what you have. A bike mechanic friend managed to repair my old college bike (about the same vintage as yours) that was long neglected, suffered being stuck out in the weather a bit . As I recall, the only thing he replaced was new tubes/tires, tape, seat, and the chain. That bike carried me on my first MS-150 like a champ, then carried a friend on her first (after I declared her current beater bike unsafe at any speed).