It will be interesting to see what happens. I am not confident that polyurethane would be strong enough to make a functional gear, but I haven't tried it. Meanwhile your project is very interesting: where did you get the steel gear you have used as a mounting for the ring gear made from the old nylon gear?
If you can find a practical solution to the gear replacement problem for the SB430 and 590, I think quite a few people will be keenly interested.
