You can get a larger than standard rotor for Novaks but not a smaller one i.e. the Novaks come with the rotor you want in them for off road but can change to one with even more torque if required.
So to confirm if you buy a Novak motor you don't have to bugger around buying different rotors for off road
More info here on 13mm rotor
http://www.shopatron.com/product/part_number=5909/135.0