Chances are, if it's still flying, the amount of original material remaining in the "authentic" plane will be about on even par with the number of original bits required to get the replica airworthy.
Seriously though, select any Mustang on the flight line, trace its lineage back to 1945 taking into account every instance of restoration, damage, rebuild, refurbishment, engine & accessory changes, parts and skin replacement, etc. It won't take long to realize that most airworthy survivors have very little of their original selves left intact. There maybe plenty of new-old-stock (NOS) parts in there, but very little that's original to that specific airframe from its days in military service. It's one of those new "ugly truths" about warbirds that's becoming more and more of a conscious understanding amidst the recent surge of the information age.
Regardless, true provenance is more of a spiritual perspective toward a particular aircraft rather than one tied directly to its physicality. Kind of like grandpa's axe - the one that's had it's handle replaced twice, and its head replaced three times.

A wise man understands that the value of the meaning centers on grandpa anyway, not the friggin' axe. Without the aces, the heroes, the stories, and the history, a P-51D would be no different than any other fighter concept that never made it past the initial stages of testing. Maybe an interesting footnote in the progress of industrial progression, but little if any appreciable admiration or emotion attached to it beyond the mundane. In the end, the value behind these original planes (or artifacts if you will) is directly relative to a persons ability to use them as a conduit to "touch" the truth of that plane's specific past. For any other purposes, the plane might just as well be a perfectly executed replica.
As for me, any warbird that I might personally decide to fly might just as well be a replica, or at least an original airframe with an otherwise uneventful history. Of course, that's
only if the type I want to fly is
available as a replica, etc.

No way do I want to be remembered as the guy who screwed the pooch and crashed the last intact combat veteran Fw-190! I'd just as soon resign the rare authentic combat-veteran aircraft to a world class museum, to stand guard over the memory of greater men than me for many centuries to come. My 21st Century "enthusiast pilot" desires have no business stepping all over all that kind of history IMO.
That said, there will always be a definite and
obvious value to keeping as many warbirds flying as is logically possible - to maintain their exposure to future generations, whether they be original or replica. An FG-1D Corsair that only flew as an otherwise "generic" Navy Rerserve aircraft during peace time is most definitely more valuable serving as a living tool, helping to expose future generations to the heartbeat of history at air shows. When the day comes that there are not ten or more FG-1D's that match that description, I'll probably need to rethink my formula. Value will always be judged on a scale of relative rarity, and those parameters seem to change on a yearly basis in the world of warbirds.
I do know this. If they ever stop flying completely, interest among new generations of prospective enthusiasts will die off in rapid succession, dragging the great soul of the tribute down with it. In the big picture, that's what "keeping them flying" is all about anyway - in my humble opinion.