Grainger. Part number 1LWK8. The motor diameter is the same as the original 6 volt motor and it is about 3/8 inch shorter than the original.
http://www.grainger.com/Grainger/ww...ing=1lwk8&submit.x=12&submit.y=9
The replacement motor has a 1/4 inch shaft and the original motor has a 5/15 inch shaft so I had to get a shim for the motor shaft.
I got the shim from American Classic Truck Parts. Part number EL116-SHIM.
The mounting studs on the new motor are just slightly more narrow than on the original motor (less than 1/8 inch) so the mounting holes will have to be elongated using a small round file.