Parts without certificate are getting harder to get through TÜV (wheels, exhaust and so on). That was easier a few years ago. They don't have to be stock, most inspectors of the TÜV don't even know whats stock on Z but it helps when they have the right measurements and look stock/old.
The historic certificate is on another level:
Any mods must have been possible +10 years of built date or older than 30 years but nobody would care about the heat shields. They are more forgiving for mods which are not visible.
Tyres at most 10mm wider than stock.
A good optical condition.
But that also means the maximum amount of tax for the car is $215. Normally dependent of the engine displacement. For example the L24: $730/yearly without historic certificate and $215 with it. Helps a lot on big V8s.