Or just go brushless. If your concerned about cost, then grab an cheapo brushless system. 13.5T (3800kv). You really get what you pay for. You can get something between $50 and $100. Don't push too much power to the wheels.
plastic does not like power. (driveshafts, hexes, shocks, etc.) Upgrading is needed when running hard and fast.
Maybe look at tires your using.
What shock setup.
There are more variables to talk about then just an motor.
You really didn't give us much to work with.

​​​​​Your probably not hooking up properly with your existing brushed system.
are you running lipo?