Servo gears will strip in a crash when the control surface (in this case, the engine) is forced back against the servo position. It's an easy fix.

1. Buy the proper gear replacement pack from your LHS.
2. Carefully open the top of the case, noting the position of all components.
3. Take out the old gears (this is why you write down the position of them).
4. Be sure you get all the little pieces of teeth out.
5. Install the new gears, referring to your notes for placement.
6. Lubricate them with a SMALL mount of white lithium grease. You can probably find this at WW in the boating or automotive department.
7. Reinstall the cover.

Dr.1