I rode my Trek 800 into a tree while goofing off for the kids. The tree won. I'm ok, but the fork of my Trek is trashed and needs replaced. What is the critical info and measurements I'll need to replace it. I'm eyeing suspension forks on Ebay.
You'll need to know how much travel you had, and try to get a fork with the same amount (20 mm more or less won't make a huge difference.)
You will also need to know the steerer tube overall length. Many used Ebay forks have the tube cut to fit their bike - and it may not fit yours. Too long is OK - you can cut to fit. Too short is too short.
Make sure the fork you get is able to fit V-brakes, as I assume your bike had. Some forks are disc brake only and have no mounting bosses for V-brakes.
I looked into putting a suspension fork on my daughter's non-suspension Trek 850 a while back. The advice I got was that the frame geometry is wrong for a suspension fork, cheap suspension forks suck and decent ones cost more than the bike is worth.