There's some debate on how much it matters, but I think it's important that the axle to crown distance of the replacement fork match that of the original suspension fork. Matching the axle to crown distance ensures that the front end of your frame is at the same height as before, so all aspects of the geometry, h-bar height, head tube and seat tube angle, pedal/ground clearance, etc remain roughly the same. If you use a standard rigid fork it'll be shorter by the amount allowed for travel in your existing fork.

If it's still in good enough shape to measure, do so, but allow for the amount it drops when you sit on the bike. If you can't measure, "suspension ready" forks are sold based on the travel of the forks they replace. You can check out some options here at the Vicious Cycles site