What laws pertain to building custom motorcycles?

It depends on the state that you are building the bike in... I would check the Department of Motor Vehicles web-site for your state to get the answer. The new rule aims to enforce the Clean Air Act by reducing the number of exempt custom motorcycles that small shops can build and individual riders can own. The rule also drastically reduces tail pipe emissions for all motorcycles effectively making carburetors a thing of the past and electronic fuel injection and catalytic converters a mainstay.