Yeah, I've fought that battle. I'm guessing that you have drum brakes front and rear. Some of the older bikes have the right hole patterns, but they won't be that narrow. You'll find a lot of WM3 (2.15) width rear rims that will work from old Ossas, Maicos...all the Spanish bikes seemed to come standard with nice Akront rims.
Be careful and bead blast to look for hidden cracks. I was all ready to mount one that ended up having a nasty looking crack on the welded seam. Sometimes it's easier to just pay the money for new rims - especially with aluminum. Most of those old dirtbikes took a beating and you may have a hard time getting the rims true. Good luck.