I forgot to post back here after all these months. I got the camera fixed!
It took me some time to decide what to do. In the meantime, I bought an F4 body for $19 (without the booster, viewfinder and back door) that had the roller with all the teeth in place. It’s still here, packed.
A friend of mine pointed me to another technician that would fix it for cheaper, so I took both bodies to him. In the end, he decided not to take both cameras apart and made the teeth with some screws or metal pins and some kind of resin.
View attachment 221464
He also did a good cleaning and adjusting and the camera is working fine. Finally, I have a fully working Nikon F4s!
Cheers,
Flavio