That's the motor unit. The broken part is the nylon or delrin spur gear at the end of the motor shaft. The gears teeth are worn to the point that they skip when the motor is trying to raise the enlarger head.
I don't have a picture of the part handy, but I removed it from the motor shaft and made some measurements:
Motor Shaft: 5/16" (measures .325")
Gear has 12 teeth, teeth are about 3/8" wide, entire gear is 3/4" wide, the flats between the gear teeth measure about .09" or so (maybe 3/32"?)
The gear is press-fitted onto a stepped (has two outside diameters) metal bushing.
The part of the bushing that is inside the gear has an OD of 1/2"
The part of the bushing that is mounted to the motor shaft has an OD of 3/4"
The inside diamter (ID) of the bushing is 5/16" and it has a setscrew to lock to the flat on the motor shaft
The bushing has a total length of about 7/8"
So perhaps if I could remove the gear from this bushing and get a replacement gear that could be press-fitted onto the bushing I would be in business?