And make sure you know whether it's 1" or 1 1/8". (Outer diameter of the steerer tube). Both exist AFAIK. Apparently, there is also some variation in the internal diameter, which determines what stem you can use with it.

I guess you can just take a tape measure to your current fork to see how long a steerer you'll need. (assuming that frame and HS will be the same). A longer steerer can be cut to fit, I presume (?), so just make sure it's not too short. Then a caliper tells you the other 2 sizes.