Well I have also the torque tube tail and exactly the same problem to fix the crown gear on the 450 size main shaft I was thinking of glue it or to drill a Jesus bolt to fix the gear.
Those were the only 2 ideas I could come up with too. I would rather go with the Jesus bolt option as I don't think I would ever fully trust glue/epoxy/whatever to hold that crown gear on. It would have to be a very small, strong bolt though. I guess a pin would be another way to go, but then you always have to worry about the pin coming out. I don't know...