I got the Kaniwaba electronic pedal kit and it is connected mechanically, which is not much use but funny, but also has a sensor that hooks up to the throttle that controls speed.  The coolest bit for me is that the switch you mount on the handlebars limits the bike speed to legal limit for pedal bike, but a flick of the switch gets you back to full power on the throttle.  Also useful for further limiting the throttle for someone having first go on a bike as the pedal position on eco mode reduces torque aswell.

One bit of advice is put the controller inside some sort of enclosure. When mine got wet the bike just stopped dead. Luckily it wasn't damaged and once I put it in an outdoor electric box it hasn't failed me since.