Does any part of the model or original box say "Sekisui" on it?
I am assuming this is a 'Con-Cor by Kato' PA, but just wanted to verify.
If it is - Which gear do you need? The cup gear that engages the motor? One of the tower gears? One of the idler gears in the trucks?
Which ever it is, I've got it. I can send it your way for a couple bucks. PM me if interested.