I found the small screws... they were in the bottom of the box (didn't fall out when I pulled out the motor).
Here's pics of the two motors:
another view:
There doesn't appear to be anything wrong with the regulator arm (not bent or damaged)... the 12 tooth gears are so small that they don't mesh properly with the teeth on the regulator arm, and after about half a gear turn one of the teeth on the regulator arm lines up directly on top of one of the teeth on the motor and the motor stops. I'd guess that it's just the wrong part rather than a design change.