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Spooky Tooth Cycles - Motorized Bicycles: Gas Powered and Electric Bikes - Home
Bicycle Engine Kit - Covert any Bicycle to a Motorised Moped. 150 MPG!! Wholesale Beach Cruisers, Chopper, Mountain, Road, Tandem and BMX Freestyle Bicycles. - BikeBerry.com
I've had direct or indirect (friend's build) dealings with all the above - and each one has turned out to be excellent!
I'd shy away from ebay etc. for a first build/buy the savings may not be worth the trouble tbh There are other reputable dealers hereabouts - I just haven't had business with them personally yet
Welcome to the addiction!