See if it has a threadless or threaded headset...probably threadless and I would bet 1 1/8" threadless. So that is the type of steer tube you need on the new fork.

Second, find out how much suspension is on your Trek y5. 80mm was common back then...what fork is on it now?

I've been checking for a fork for my 1996 Specialized Stumpjumper and came across a RockShox 30 Gold TK fork that might fit the bill for me. It would be a 100mm suspension fork replacing the 80mm suspension fork, but I'm thinking it may work fine. I will have to replace the front wheel with a disk brake wheel, but that isn't all that big of a deal for me.

I'm interested in other poster's opinions on this subject too.