This is a very delicate subject. In California, they allow customization of cars provided you pass emission testing at a referee station. In Illinois, a car can be exempted as a show car. This is by definition something you drive 1,000 miles per year and only to restricted activities. The Federal Law on this subject states that all original equipment emission controls must remain on the vehicle. However, the law was never intended to cover vehicles over 100,000 miles use. It was assumed that these vehicles were be minimally used. California, New Jersey?, New York? and some other states have exempted 25 year and old cars from emission testing. This basically allows changing of the drive train. Check with you own state for exemptions. Dr. Bob