You can send it in for service. When I have sent stuff in they only charge for exactly the parts that need to be replaced, and at a reduced rate. So if it only needs one gear, you might not have to pay for a whole gearset.
From a post on Radio Control Zone:
Global Hobby now doing Airtronics service
Ship repairs to:
18480 Bandilier Circle
Fountain Valley, CA 92708
tel: 714 963 0329
fax: 714 964 6236