I have an older 1990-1996 Shimano Mountain Track 800 that I got from my brother and is in pretty good condition. The past couple months I've been going thru and upgrading all the parts to make it a little more comfortable to ride on the road, and just doing some general maintenance. I'd really like to replace the crankset, because its the same one thats been on it for 15 years, and its a little worse for wear. Now the stock Bottom Bracket is a Shimano square taper, and the crankset I have in mind uses an ISIS BB. I've already popped the BB off, but before I run out and buy a new one, I wanted to make sure that if I got the properly sized ISIS bracket it would fit. I'm not clueless when it comes to bike repair, but neither am I terribly experienced, so I wanted to just make sure everything would work before I started throwing money around.

ISIS is a dead BB type, you can get replacement BB's at the moment, but no MTB cranks have been made in this format for years.

If looking to replace a BB and crank today, would be looking at an External BB solution, or sticking with square taper.

Whatever you choose, would look at the cost of the repairs / replacements vs a new bike, especially as you are working on a 20ish year old bike.

Trek 800 ? Pretty lowend bike to throw money at. I wouldn't swap cranks or BB until they actually broke. I'd just stick with square taper UN 26, 54 or 73

Acera or Deore cranks from Niagara cycle.

But only when necessary.

With a little patience you could get a nice Trek 930 complete off CL for the cost of BB+cranks. That's a better bang for buck upgrade than what you're considering.

If you go to vintage-trek.com you could probablt nail down the year of your bike