Thorsten, in Austria an "Einzelgenehmigung" would be the way to go.
(The CoC papers do not cope with the actual car!)
In case of an accident the insurance company could refuse payment or even insurance coverage as the car is different to the registered specification: Wheels, brakes, engine power, emissions, noise etc.etc.
The German authorities are quite strict, so be very careful to have the right specifications in the Fahrzeugbrief/Typenschein.
