[1985 CCX]

Newer classics can be race registered just very detailed highly accurate versions in period trim.
For example there are at least three FIA Kirkhams in the process of acceptance it just requires details such as Magnesium wheels, steel tube suspension, Girling brakes basically all old school period stuff.
The US is more forgiving. John is correct as the field thins more forgiving rules will be made to keep things alive or our kids will never see this first hand again.