You could install a 90mm band brake onto the rear wheel since it has existing M34X1.00 threads. However this would be a custom installation job which would most likely require cutting the axle spacer in the right place to mount the band brake housing onto the axle. The band brake housing needs to be sandwiched between two axle spacers in the correct place in order for the band brake to work. The toughest part of this job would be cutting the spacer straight. A metal chopsaw with a thin blade would do a good job of this, and it could also be done with a hack saw if it was cut very precisely and then sanded straight afterwards.
Rear disc brakes could also be installed on the rear wheel however this would require custom metal fabrication and welding. All of the parts are available at ElectricScooterParts.com for this job such as disc brake rotor adapters with M34X1.00 threads, disk brake rotors, and disc brake calipers. A mounting boss would need to be fabricated and welded onto the frame of the scooter to hold the caliper in place. A full service bicycle repair shop could assist with fabricating and welding the mounting boss for the caliper.
I have a eZip 750 electric scooter. It has only front caliper brakes. The rear wheel has threads to put band brakes. I want to put rear brakes on it, what parts do I need to order from you? Could the 90mm band brakes fit?
Also, is it possible to put rear disc brakes on this? If so what parts do I need to complete this?