You could look up the 60mm travel forks of the day (did it have a Judy?) and check the axle to crown.
To preserve geometry, you want a rigid fork to have a similar axle to crown as the fork would have when properly sagged. Of course, you can go to a longer a-t-c to slacken out the bike and raise the bottom bracket, or even a shorter one to quicken up the steering. Most people would choose the former option with an older bike. Mountain bikes have generally gotten a bit slacker over the years to increase technical capabilities. Suspension travel has often increased as well, and you won't really find new forks with more than 80mm travel.
But 80mm would probably work pretty well, and there are lots of good XC race forks that would be a great match for that frame. Just depends on your budget and preferences (lightweight? Stiff?).