From the photo, I can't divine too much about the strap, but it does appear a shade long for the helmet.
However, I once purchased a Kugelhelm alleged to be "complete with original leather chinstrap" from this seller.
Opinions vary, but I don't place much faith in what I received: "picture frame" flat buckles and modern leather "aged" by boiling it in water and drying it in an oven. Ugh!