From what I could tell,very little indeed was used from the wreckage of the spitfire.It seems the big issue is establishing and having ownership of the identity of the original plane.Once you have that it appears that you can replace any number of parts (which in this case even included the fuselage),and it is still recognized as being the original aircraft.Apparently it also makes a difference when it comes to matters like approval etc...as it counts as a restoration/rebuild.But like you suggest it does seem to be pretty much a new build.They did a fantastic job on it tho didn't they.Brilliant engineering.