Bionx makes motors and battery packs that most people probably won't be able to recognize.

If you get the 350W, 36V lithium ion powered version, they claim you can get 46 miles by supplementing your pedaling with 50% electrical assist.

Having said that, few people will be fooled even if they're not aware of the motor. Plus, there ain't no satisfaction in cheating. Getting electrical assist to try to convince people you're faster is like taking a shortcut on a century.