It is most likely to be the speed controller which has failed, however, there is a smaller chance that it could also be the throttle.
The throttle can be tested to determine if it or the controller is the part which has failed. Please see the throttle test directions below and also attached.
The exposed sensor leads do not look broken or short-circuited together so they do not look likely to be the culprit. If the throttle is faulty the reason is most likely to be the sensor though.
If the throttle's lights are on when the power switch is on then the fuse and switch have to be good.
One thing I just thought of is that it could be the switch in the brake lever. I would disconnect the brake lever's electrical cable from the controller and see if that helps.
Also, even though the batteries have been replaced regularly, I would not write them off since the motor works and the throttle and controller are new.
I would check the lights on the throttle and make sure all of them are on when the throttle is at the zero position, and then watch the lights when the throttle is turned to the full throttle position to see if any of them turn off. If any of the lights turn off at full throttle that could indicate a problem with the battery pack.
Scooter been running strong for 6 years. Replaced batteries regularly. Suddenly doesn't show lights on voltage display. Won't do anything. Fuse was blown and plastic clip wire to motor slightly melted. Replaced both. Now will show power with charger hooked up only and 2 clicks while engaging the throttle. Motor runs if wired directly to any one of the batteries. I don't think it's the batteries or motor. Flipping between speed controller or throttle. Any help?