http://www.motopeds.com/
I found it in a discussion about such bikes on a motorcycle forum. Maybe it was ThumperTalk, I don't recall.
You buy the parts, build it yourself, and get it licensed your self. The risk that you can get it licensed and running is all on you.
There are a number of motorized bicycle setups out there, but this one seemed like it would fit into my lifestyle more than the others. It isn't really a motorized bicycle as much as it is a hybrid somewhere between a motorized bicycle and a moped. A little research indicates that in most locales it would legally be a moped, not a motorized bicycle. In WA state you probably could not ride it on 'public' bicycle trails even with the engine off, and you could not ride it on a 'restricted access' highway and it is not supposed to exceed 2.5 HP or 30 MPH. Your laws may vary.
It is fairly heavy for a motorized bicycle, over 100 pounds where others weigh much less, but also are just bicycles with a small engine somewhere on them. This is more of a moped - a motorcycle with pedals.
Still, a 100 pound motorcycle is very light - most motorcycles weigh well over 200 pounds. Mine weighs 280 ready to ride. Even trials bikes are rarely lighter than 200 pounds.
But, if you got a nice quiet silencer on it (I kind of doubt the one shown is quiet) then it would be good for putting around a campground, going down to a store to get some groceries or just to carry along as emergency transportation.
There are some vids on youtube.