One of our club member's U33 had a failure where the square piece (which was hanging down in your picture) broke off the rest of the unit. The part came from Lionel and we fixed it at the same show the next week. This involved removing the truck, unplugging a JST connector for the pickups in the truck, and fitting the new U-joint, bushing and gear into the truck. the shaft. Reassemble the truck and insert the square portion into the female receiver on the motor. Re-attach the electrical connector and then the truck to the chassis.
I'm not sure what the female portion on the motor shaft looks like or how it could fail. If the pieces just slipped apart the female portion on the motor shaft may be too far away from the truck and it came out in a curve. Sliding the female receiver a little (emphasis on a little) away from the motor and closer to the truck might be the solution - no parts needed. You may not even need to remove the truck to do this if you are able to assemble the u-joint and then adjust the female portion.
As memory serves, I removed the fuel tank thinking it was necessary to do this. It turned out to be the most difficult part of the reassembly because of the location of the screws. I didn't have to access the motor shaft, but as you may have to, take pictures as you go. Make sure you have a good set of precision screwdrivers handy. The other thing I seem to remember is that you have to plug in the JST connector with the truck really close to the chassis, as the wires are very short. This must be done with tweezers. If you don't have to remove the truck you won't have to worry about this. If you do... take your time.
After you reassemble, run the loco through your smallest curve with as little ambient noise as possible, listening for any roughness in the u-joint.