There may be some variation among brands and how the spring is retained, but shaft is in the 4" neighborhood measured from the outside face of the exhaust air duct shown in Russ' pictures. Shaft is 5/8" and has a long keyway. It looks like you don't have a motor pan, and sounds like you'd also need a spring and the retaining hardware. That 'auction web site' often has motors complete with sheaves, motor pan, often a power switch and cord. I looked earlier and saw only 1-1/8 HP, which is an upgrade but pricier than 3/4 HP. Complete headstock is a good idea if you find one, the poly-V system is quieter and you can find one with a two-bearing quill if you want that. I've noticed shipping varies a lot on complete headstocks.