Gas Gas Electric
In Austria December 2012 I saw a factory electric Gas Gas working obstacles for a few hours while we gassers were punting around, and the kid riding it was better than all of us. Maybe the time is limited by the battery, but he did not appear to run low. The battery and engine were a nice small package and the bike was said to weigh around 125 pounds. Maybe the power is described as comparable to a 125, but I saw it blast nicely up verticals where it would take a lot of effort on a 125. I want one of these if they ever come to the U.S., but would be afraid of a homemade one.
They creep up on you without the sound of an engine. If your bike is running, you will never hear an electric approaching.