you could buy a new cassette and a new rear D while you are at it get a new shifter -and move up to 7 speeds. BUT...
30 years ago is a Loooong time for mountain bikes. you might do better to find a used bike that is just a few years old with less problems - it would probably weigh 20 pounds less that your old bike.
you could fix up your old bike but My personal opinion is your money would be better served buying a modest modern bike.