If you are looking at motor upgrades, take the advice above on axle upgrades and metal gears, brass gears are ok, but wear quickly.
For a robust setup steel gears, metal bearings and for any setup where you take drive off one axle to power another, consider the upgrade drive thru type axles. That helps eliminate or at least reduce torque twist.
I have designed many motor setups for WPL trucks ranging from dual 180 motors, dual 370 motors using electronic differential, or dual 370 motors with transfer box, 370 single & double belt drive setups.
My wife & I run WCP models and we have loads of motor options available.
They are tried & tested aswell, we do send some out for review & testing purposes also.
If you want scale power & torque and quiet drive we have dual 370 motors. They let you haul a serious load.
However out new single 370 motor setup now has a reduction gearbox which makes it awesome for offroading and is compact and more lightweight than carrying dual motors.
We also do commision work if you have idea or special project to work on
Some videos to show off different setups
Dual 180s electronic diff (12kg oil filled radiator)
Dual 370 electronic diff
Dual 370 motors with metal gear transfer case
Word of warning, WPL axles with serious torque are dead easy to break. If you hang up a wheel and add power, dual motors like ones above can shear, twist and break metal driveshafts of all designs
We have also run WPL's on 540 brushless motors and 700 size motors and although funny, are mostly pointless.
370 motors are ideally suited for this scale of vehicle
The 2 speed gearboxes are fun, but the plastic gears dont last long, metal ones are available however, but they take up alot of space and heavily reduce ground clearance