This is my rear derailleur (Suntour AG-5000 1983) and Maillard France freewheel dated 1983 still working after 33 years and og shimano uniglide MTB chain 3/32x1/2 (date code 3F 1980)
Clean and lube the derailleurs front and rear while you have it apart, and KMC makes a good chain and very affordable all bike shops carry them or Sun-Lite brand
Or find a donor bike with some used good condition rims with a cassete instead of a freewheel ? Which ever will work and provide you with many enjoyable bike rides as my bike is a freewheel and works just fine