How I chose my motor, was just to go on Austin EV and look to see what everyone else was using. Then I just found what was appropriate, and went to find it used somewhere.
I ended up buying a used ETEK motor for my cycle.
AustinEV's "sort by motor" shows 688 series wound motors, the most popular selection, followed by 292 Permanent magnet DC motors, which is what mine is.
http://www.evalbum.com/mtrtp