+1 on the warnings. It is not a trivial issue. I would look for a fork that has the equivalent CR to Axle distance as your suspension fork with sag has.
I have a 97 RockHopper that was offered with suspension. But this one was rigid. Here is a pic that demonstrates that there are forks that have the distance built in so the frame could be the same for suspension or not. The tires are 2.1 x26, note the clearance:
I mounted fenders on this set up and had to make an extension to mount to the fork!
