Scratchbuilt, yes... but he's not making that internal stuff up out of thin air. He must have an incredibly detailed set of plans from somewhere that he's working from.
Harry, virtually all American aircraft manufacturer's engineering drawings since the very early 1930's exist someplace. For example,Boeing Aerospace (who also bought out North American [builder of the P-51 Mustang] and Vought-Sikorsky [Vought produced the F4U Corsair series]) maintains a very complete historical archive of their aircraft drawings, as well as drawings from the companies they absorbed, and they are available for a price. The same is true of Lockheed and Northrop-Grumman. Many of these drawings have been published in aircraft enthusiast magazines over the years as well.