You never have to separately register a motorcycle frame in the United States. If you are going into the custom motorcycle frame business you should stamp each frame with a serial number of your own devising and ship the frames with a "Manufacturer's Certificate of Origin" stating that number and when and where you built it. Any frame you build will, by definition, be part of a "special construction" motorcycle. All homemade choppers and kit bikes registered in the United States are considered to be "special constructions." Technically, any registered motorcycle with mismatched serial numbers on the frame and the engine is a special construction. The process of registering a special construction generally follows a few steps.
Register a motorcycle with a headlight with a high and low beam, a horn, a left side mirror, fenders and front and rear directional signals. Register a motorcycle that the laws of your state or province consider to be legally "road worthy."
Present a Manufacturer's Statement of Origin, or MSO, for your frame, engine and transmission to the Department of Motor Vehicles for the state or province in which you live. Present receipts for and proof that you have paid the local sales tax or you will be charged sales tax for the component parts.
Present your custom construction motorcycle to the motor vehicles department or the state police, depending on where you live, for a road worthiness inspection.
Insure your motorcycle with a recognised motorcycle insurance company in your state or province in order to complete your registration.