Since a gear often broke in my gearbox, another solution had to be found. after all, I didn't want to constantly repair the gearbox in the field ...
I found this gear on the internet: -click-

and these drive shafts:
-click-

Although these are made of plastic, they are very stable. also variable in length so that they also work with various wheelbases.
Now the gearbox only had to be transplanted into the chassis.
I designed a central gearbox mount and two adapted chassis tops for the wheelbase of 290mm that I used. the front upper part of the chassis got a box for the receiver (graupner gr-12). to cool the motor, i put a small fan on top.
with the two battery holders and my brushless motor (3650, 2300kv), batteries up to max. 139x48mm possible.
I had calculated the total gear reduction, but unfortunately forgot (maybe "22: 1" ???). with my motor and 2S lipo are min. 20 km / h possible, with 3s correspondingly more. enough to move through the terrain accordingly. and that with moderate electricity consumption. with a 2s / 5200 lipo I'm at about 60% throttle min. 45min on the way.
PS: I did my adjustments to the chassis with tinkercad. So not everything is perfect, but it does the job.
one more thing: I blocked my rear diff with hot glue :-)
if you have any questions, feel free to ask me ...