That's beginning to be a problem lately. I do some autocross, and the tech guy is getting strict. You try to explain that the car has been out of production for almost 40 years, but the tech guy just says, "It's not stock, according to the SCCA rules" to your aftermarket part, and he puts your stock S30 into a more expensive class, like ESP (E Street Prepared.)So either you build a full-out competition car to compete in the modified class, or build a time machine to get factory parts, or you buy a new BMW, to go to the local autocross. But for regular street driving, I would just go with whatever will make the vehicle functional, reliable, and safe, and still allows you to maintain the Z-ness. Most spectators don't know the difference, or will mis-interpret the part as a mid-model-year correction of some sort (the casting lines on the intake are supposed to be "corrective welds" so I was once told.)