Myself, I went for 45 km/h officially (unoffially, on a flat road, I could reach 53 km/h). While turning, for safety reasons, I limited myself to far lower speeds (25 km/h).

Designing a car suspension system for reasonably high speed seems hard, I have never tried, instead choosing the robust and crude solutions to get a reasonable assurance of strength.

Motorcycles seem easier. Especially since most of factory-made motorcycles use a sprocket and chain - a very flexible system for dropping in other power sources. I imagine that with enough know-how to get through type certification, a lot of combustion bikes could become e-bikes with excellent riding characteristics. :)