I can’t imagine there are that many new kit cars going on the road each year that would even be worth the watermelons interest. Besides, most guys that are building such things are going to put a modern powerplant in it that at worst lacks cats. Then it gets driven 3K miles a year, if that.
Also, even old cars that are restored to original can easily be converted to run on the crap gas and thanks to better piston rings, bearings, motor oils, etc actually run cleaner and get better gas mileage than the day they were driven off the showroom floor.