I would say that this Kurassierhelm was original except for the chinstrap.
The Kokarden look okay to me. Both the M15 Kurassierhelm and the M15 JzP helmet can be found with 63mm or 48mm Kokarden. Both are completely acceptable. The National Kokarde shown looks okay in the photos. It should be thin stamped steel.
This helmet retains more grey paint on the trim than is usually found.
John, I respectfully disagree. That you find many Kurassier and JzP helmets with small cockades does not mean this is legit, and historically acceptable. The chance to find these helmets with their original cockades is low, and because these large cockades are scarce, they have been commonly, and wrongly, replaced with small ones. In addition, I am almost convinced that the small cockade on woznotwos' helmet is a reproduction: sharp "teeth" and red colour that is not quite right.
I tend to agree with Bruno. We should make a lot of picture searches. It is incredibly interesting question. Especially for the helmets that came back to the USA in 1918 and 1919. We have a lot of reason to believe that many of the war bond helmets were literally slapped together.