$50 isn’t much to cover a BB, RD and maybe more(?)

I can’t say whether the Roadmaster BB would fit, but it’s likely a low quality part and the Stumpy is a high end bike. It deserves better.

If if there is a co-op near you, that’s the way to go.

As as far as resale, the market has generally tanked for vintage bikes, but it’s a Stumpjumper and they have a following. People like the rigid ones for all-rounders, but I am guessing yours has a suspension fork? All the same, if you buy it cheap enough you should at least be able to recoup your investment.