What if you get a car that has no exhaust...or, no engine/drive train and the owner goes from there, down to the shop, like yours, where do you stand leagally?
As an installer of exhaust I would have to make it compliant to the year of the auto. But, if the car was brought to me on a trailer with no license plates and I was told the car was going to be a race car and was not intended for "on-road" use I can legally put any exhaust system on the car and still be within the law.