Actually, that bike might be one of the easiest to derestrict if you're any good at electronics, especially if it has the standard 9 pin connector to the motor.
There's no separate external speed sensor so it must be in the hub. Assuming it's a hall sensor motor, but even if not, there will be two wires carrying 5v to it and one bringing the speed signal back. You could get to them in the controller compartment under the battery or make a short adapter lead with the male and female motor connectors as long as they're standard.
When you have access to those wires, you can use any microprocessor to reprocess the signal to a lower frequency, something like am Arduino nano, or you can use a dual D-type flipflop as explained here:
http://www.electronics-tutorials.ws/sequential/seq_4.html
Forum member Festivejelly did that successfully and said that he was going to make a 3-D printed case and complete schematic for it. You should send him a message. Maybe he has a complete solution.