Not disagreeing with the above post, however in the scenario where you do get defected by the popos (revenue raising) chances are you won't bother arguing about eng certs (needless to say the time and money spent to obtain one) and you will just put your stock seats back in, get a rwc and have it cleared (face it, it's just easier, and they will prob dick you for other things too).

Moreso, why would you risk your life with a replica seat? it's the only thing holding you in place, some laws are arguably stupid however running replica seats is a real safety concern for the driver/passengers of the vehicle.