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

The bike is in reasonable shape, except the rear wheel. It has seen its
share of abuse. There are 3 spots of zero treads on an otherwise good
tire (original?), and the wheel can't be trued. The rear bearings has a
bit of grind to it. It looks like someone took it out for some rough
riding and curb hopping.

I've been out of biking for a couple of decades. It seems that now
there are two types of gear clusters- the "freewheel" from the 80's, and
a newer cassette design. The mtb I inherited has a Shimano MF-HG20
cluster/freewheel.

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?