In a couple of earlier posts (Bike Trailers, More Bike Trailers, Electric and Cargo Bikes, and Sport Utility Bicycles ) I wrote about ways to make your bicycle more than just a machine for the occasional spin around the block. A couple of days ago I ran into a site that sells small 2 and 4 cycle engine kits to make your bike into a mo-ped.
These conversions are not nearly as expensive as an electric motor and battery system would be, and look like so much fun that I'm thinking hard on getting one.
Legally (in Utah), motorizing a bicycle requires that you have a motorcycle endorsement on your drivers license.... and I have no idea whether or not you would need to register it as a motor vehicle or not....I'm thinking they probably wouldn't make a big deal out of it unless you went into the bike-to-moped conversion business.
Here is the engine buyer's guide page.... the 4 stroke 49 cc engine kit looks the best to me. Not that I don't have enough to do already. I just don't know if I will be able to resist.
Have fun. There is a ton of information on these....I had no idea. Also, look at motorbicycling.com - they have a discussion forum with a lot of information.