Here's another electric conversion that looks like a pretty cool project that isn't too difficult and has decent performance. Starting with a fat tire beach cruiser bicycle they turned it into a 48 volt electric motorcycle.
According to the builder it will go a little better than 45 mph and if cruising at a moderate 25 or 30 mph it will run for a couple of hours.
They have step by step details of how they did it and a parts list if you would like to give it a try. Pretty neat. I like the "V-Twin" batteries, too.