Good morning Eric,
In 2017, I had a similar happening with the bow thruster.
High pitch sound and no propeller turning….
The gear also did not look that bad.
Previously I had purchased from Amel the Tufnol shaft, but the one I received was not the one I had on the boat…. Pictures I sent to Amel at the time:
Maud told me at the time, that they did (from memory) 3 models/size/gear dimensions.
So, I opt to purchase the complete kit: Amel part 1110 - called in French “MECANISME PIED DE PROPULSEUR SM”
Which you can see at the bottom of the page:
When I received it, I was a bit disappointed that I had to re-machine one of the gear to fit the key.
So back to your issue, I believe that Amel sent you the wrong gear dimensions.
Either measure the gear to have to give them, or purchase the complete kit (small and big gear).
Hope that help, sincerely, Alexandre
Currently visiting Switzerland.