No probs keeping a Mickey Mouse one original, but i'd still swap the driveline out. It wouldn't devalue it in any way, and it is all unboltable anyway. To me that is still original, just had the driveline removed and put on a pallet. The only thing probably sacrificed would be the exhaust but it is unlikely that would be all original anyway, want to be a pretty good car to still have an original exhaust - my HJ Prem does but not the muffler.

If you had to rebuild one you'd be wasting money rebuilding the 6cyl driveline.