I don't think the speed would be an enormous problem; the legal (powered) speed limit for e-bikes in the UK is essentially the same as when the C5 was new, at 25km/h. So as long as you stick with a stock EU/UK standard controller you should be fine.

And you'd get a fantastic amount more grunt uphill and a longer range with a modern battery and motor.

Could the car lean if it had electric suspension? Is this a fundamental limitation of a 3 wheeler with 2 in the back or is it just difficult to achieve?