For your own safety, do not ride a motorised bicycle with rim brakes, and do not ride a motorised bicycle without the largest front disk rotor you can get your hands on - end of story.
If only SickBikeParts or similar engineering company made a 12 inch front disk rotor and caliper adapter to go with their motorised bicycle kits, it would properly address the need for efficient braking with respect to a motorised bicycle application.
I would buy the first person to purchase a proper braking upgrade kit if it were made available, considering i am using a 9 inch front disk rotor, and it barely does the job.
Using rim brakes is simply suicidal...