so far the grid results to be NOT on the "approval certificate" of the bike.
so we have two scenarios:
1) German one (pls correct me if I'm wrong): i.e. any parts even if aftermarket, is approved for road use, than it can fit any bike (see tyres, wheels, exhaust) without any bike producer "nulla-osta" (compliance)
2) Italian (for example) everything not mentioned in Bike Certificate, is simply forbidden.... for istance... tyres ,wheels, brakes, even the main bike dimension as lenght or beam (imagine to fit sidecases or wide handle bar) must be written into bike certificate (fyg in mine I have two beams, one with and one without sidecases, I cannot fit sidecases wider than what is written.