Simple then no single race car is "Original" then.
For it to be historic/classic it must be at least....checking MSNZ rules....15 years old (for T&C - don't ask me a car that is 15 years old is not classic in my mind and never will be - just an opinion and we all have them but pre 78 for me = classic) and some part would have been replaced in that time.
So using the dictionary definition there is not a single "Original" Classic race car around on the track (sure there might be in a museum/collection somewhere but would you want to trust it on the track with all the "original" gear, pads, plug leads, tyres etc)