A friend of mine looked into building a P-51 from scratch, as buying a real one was too expensive. He ordered the blueprints, which are readily available. But then he talked to some P-51 owners, who told him the blueprints wouldn't be much use. The airplane was defined by the jigs used to build it, and the blueprints were just drawn up later as documentation and never checked. The jigs were scrapped when the airplane went out of production.