hi john,
can i offer 2 suggestions to help preserve your gearboxes, I assume it is the plastic rear plate on the gearbox that holds the motor on that is your problem??

1- use a second, or move your current, large hole mount (the one for the rear of the motor) to the other side of the ferrite ring (you need to slide this off the motor and replace later) which gives more support at this point. use screww/bolts and thin ali or steel tube as a long washer to brace this area for extra rigidity.

2- drill holes in your wheels tire. assuming they are solid rubber, putting holes across the tread creates squishy spokes in the rubber. these can be tuned with a little trial and error to give support running normally, but under the high loads of flipping, will deform and allow your chassis to rest on the ground, taking the loads away from your gearbox.

hope this helps
james