I'm fairly certain that all gear boxes are sold with the motor attached. I doubt you'll be able to find a replacement gear in a hobby set because of all the variables. I mean, not only does it have to be the right thickness and the right diameter it has to be the right pitch and have the right number of teeth and on and on. I'm assuming that the gears in the tank now are made of plastic so I would recommend that you upgrade to metal gears. I'm not sure exactly what fits in the vs tanks but Erik might be able to help you with that.