Preferably you should match the following:
-type of headset, threaded or not
-size of headset, 1" JIS/ISO or 1 1/8".
-length of steerer tube
-axle-to-crown length
-rake (distance between an extension of a line through the center of the steerer tube and where the wheel axle will sit.)

Just about any road bike fork for the intended wheel diameter that'll match the headset will result in a rideable bike, but the bigger the changes in rake, the bigger the changes in ride characteristics.