Join Date: May 2009
Bikes: Salsa Fargo, One-One Inbred 29er, Blue Norcross
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You need the ERD (Effective Rim Diameter), the drilling (how many spokes there are) and probably should stick with something at roughly the same width. The ERD is basically the inner diameter of the rim, and with hub flange height and spoke crossing determines how long of spokes you need.
ERD is a published value, but if you can't find it for that particular rim, you should be able to measure it. From the sound of things, your front rim, but not the back is damaged, correct? If so, measure the rear rim's inner diameter (where the spokes enter the rim) at several places and take the average, then add 1-2mm since you didn't account for the thickness of the aluminum that comprises the wheel.
Once you have the ERD, you can find a rim that has the same (or very similar) ERD. There is a certain amount of fudge factor with ERD and spokes, so you can usually go 1-2mm either way and still use the same spokes, especially with double-wall rims.
I would recommend heading over to Sheldon Brown and doing some reading on wheel building so you have an idea of the mechanics involved with wheel trueing before you launch into this project. Good luck!