Maintenance item replacement on an original car is entirely acceptable. As we all know, sitting disease is the worst for a car and keeping it running and moving is much better, hence the latitude in wear items by originality/preservation judges.
With any car, consistency across platforms is the overarching concern. That is, does the exterior, interior, engine bay, trunk, top, and undercarriage match? A worn car with a great paint job looks silly, as does a beautiful car with a ragged engine bay. Within preservation classes, that consistency is watched for as well as overall condition. Obviously, the best condition, most consistent, oldest and most significant (rarity, style, d'elegance, etc.) car wins. Within a particular model class preservation judging, like our cars, the best condition, most consistent, least altered example wins.
Here's more information from a nationally recognized survivor/preservation judging event. http://www.survivorcollectorcar.com/ Not all old cars are original/preservation candidates which is what makes them so collectible.