Amends the Clean Air Act to require the Administrator of the Environmental Protection Agency (EPA) to waive requirements for recertification of kits for the conversion of vehicles into vehicles that are powered by natural gas or liquefied petroleum gas in any case in which: (1) the kit has been previously certified for the model of vehicle to be converted; and (2) neither the kit nor the design and specifications of the model of vehicle to be converted have substantially changed. Requires the Administrator to waive emission certification system requirements for a vehicle that is more than 10 years old or has more than 120,000 miles and that is powered by natural gas or liquefied petroleum gas.