That is the whole issue really, there are no RULES.
What he have is a world wide debate with different opinions, like A-holes, everyone has one.
PERSONALLY, they should be as close as possible to the original. If there are no components PROVEN to be from the original, they are tributes, copies or fakes.
If they have 3 of the major components from engine, chassis, body, driveline or wheels (ie 3 out of 5) they are a rebuild/restore. This does NOT mean copies of the original bits, it is the genuine items.
Any less than 3/5 then they can not be considered as anything better than tributes as someone else is able to make another with at least the same number.
This is not my original thinking, it is just what Jenks said, and I just happen to agree.