Catching his breath alongside a road near Seattle, WA USA
1999 K2 OzM, 2001 Aegis Aro Svelte
The important measurements are:
Steerer diameter - This is most likely 1-1/8" but you'll want to check to be sure.
Steerer length - I'm going to assume you have a threadless headset and most threadless steerers come long enough but will probably need to get it cut down once you've settled on a proper handlebar/stem height. For threaded steerers, you'll need to pick a fork with a steerer that's close in length as to what you have. I guess you can always cut down a threaded steerer too.
AtC (Axle-to-Crown) - this will need to be measured and you'll want to match the appropriate suspension correction on the rigid fork to what you currently have in order to not effect the handling on the bike.
Fork offset - again, you'll want to match this as close to possible so as not to change your steering characteristics
Other things you'll of course want to make sure are correct include brake bosses. You probably need cantilever bosses but if your bike has calipers then you'll need a fork with the proper mounting hole and if you have disc brakes then you'll need a fork with disc tabs. Your best bet is to take this to a shop. You'll need them to help with some of the installation anyways as you'll have to pull the crown race off the old fork and stick it on the new one. While that can be done with jury-rigged tools, you might find it easier to have the shop do it for you. They can also cut the steerer and help in the transfer of the brakes.