Ok, so I have drawn it’s replacement and sent it off for 3d printing. First time I have done either so no idea how it’s going to turn out.
Mostly those gear wheel are broken by debris and hairs.
A skilled person can change these easily,
THANK YOU FOR THAT LINK!
You can see here a broken tooth caused a stripped gear on a 30C. And after only 6 months of use!
Made a request to the rushgears site (thanks) but hopefully can find something sooner. For now, Eufy is running much quieter with only one brush.
I think such a gear can be found as it should be nothing exotic.
Before you replace it : GREASE!