I was looking at the 48V 1KW Regenerative Braking motor controller than Golden Motor offers.
But they won't answer any questions about whether or not it could handle 60 volts. The stock controller for this scooter has motor braking, but it doesn't utilize the current to recharge the batteries.
Golden Motor also offers a 48V 1KW hub motor in a 16" cast aluminum rim that could be a direct replacement for my current one.
A 1KW motor and 1KW motor would result in having a 24 MPH top speed on level ground at only 48V instead of 60V. At higher voltages it would probably end up at a non-legal speed.
But, I think I want to continue on the path of having high voltage and low amperage. I think that will net me a much higher total range, and I'll have the performance figures I'll be happier with (29mph on level ground, 25MPH uphill, 35-40 miles range).
When I do go to 72V I'll probably mount the charger(s) on the scooter.