If you are friction shifting, virtually all rear derailleurs will work as long as 1) the derailleur has enough throw to reach all the way across the freewheel or cassette. (Later SunTour Cyclones handle 9-speed cassettes with ease) and 2) your shift levers can handle the cable pull amounts. This limits you to nearly every derailleur made after 1989 and many before. (Pick up a box of used derailleurs and maybe one or two will be incompatible. Friction shifters are a joy, especially here!)

Shift levers with bigger drums will take up more cable and handle more cable pull at the expense of being harder to shift accurately and requiring more skill.

Very worn pulleys work just fine, if you are not using index shifters.

Ben