Not my decision to make, and opinions will clearly vary significant. There are people in the classic car market that would call a car non original if it had literally one part that wasn't what the car had coming out of the factory, for example. They even believe the faded, practically non existant original paint is a million times more preferable to the car ever having been touched up or respayed. Then there are others who are far more pragmatic about it.
But if one were to take a rational logical view on the question it would be reasonable to put a figure of 50% being the limiting factor to be able to claim if something is still original or not.