Most parts places will sell the entire wiper motor assembly, not individual parts but we found an "as is" wiper assembly for a fraction of the cost. The plan was to tear them both apart and cobble working parts together.
Once we had both gear boxes apart and the gears pulled, we suddenly realized that the threads ran in the opposite direction. This meant the plastic gear would not mesh with the worm drive from our electric motor.
We had already established that the "new" motor would not run as it was too rusty. So we took the two motors apart and swapped out the magneto and worm drive. It needed a little cleaning but worked!
This same wiper assembly could be recreated with Legos and would provide great lessons on mechanics: taking circular motion, turning it 90 degrees and converting it into an oscillating motion. I used the example of a steam engine drive piston when explaining the wiper motor's mechanism.