Here is an electric bike from Currie Technologies called Ezip on sale for $299 at ToysRus.com.
It looks like it has a 10-15 mile range. It is 450 watts and goes up to 18 mph.
Of course, you could take this motor shown above and replace your existing motor for double the power to 900 watts instead of 450 watts. This would half the range, but you could add a second battery pack.
Or you could make your own from scratch. Check this video out.