It certainly is possible to mate the motors with the gearboxes but you'll likely need to find a pinion gear for the motor and make sure that the mounting holes in the gearbox fits onto your motors.

Yeah, RC car stuff at this level will strip fairly quickly (as do some drill boxes I have to say, try and find ones that are all metal if you can). If you had the time, you could build your own 1 or 2 stage spur gearbox but that can get fiddly.