Veggie wrote:
I intercepted my in-law's bike on the way to a yard sale. It's
basically a clone mtb circa 1985.

Can I drop a modern wheel into the bike and expect it to work? Will
there be a gear tooth spacing that can cause problems?

It' do-able. You can take the old freewheel off and put it on a new
freewheel hub wheel or get a new cassette type wheel. Only problem with
the latter option might be chain and chainwheel wear.