HalcyonDaze,
Depending upon your motor shaft diameter, the pulley is either a 9-428 (1/2" bore) or a 10-428 (5/8" bore). The same pulley is used on the Atlas (and Craftsman) 10" and 12" lathes. If you are looking for an original, you'll probably have better luck checking the lathe ads than the mill ones. I'm pretty sure that you can still buy the 5/8" bore one from Clausing, although I don't know the price.
If you end up having to make one, in the Yahoo atlas_craftsman and atlas_craftsman_projects Files sections you'll find some information on pulley making and a drawing for making a 10-428. According to that drawing, the two diameters are 4.390" and 1.930". I think that the drawing was reverse-engineered so all of the dimensions may not be exactly the same as those on the Atlas 10-428 drawing. If you need the drawing, its on the _projects list under Drawings, Atlas or Atlas-Craftsman Machine Parts. If you can't get to it, let me know. The pulley making info is on the main list.
Robert D.