I had YAMAHA HP-1 with the broken head strap. I wanted to repair this headphone too, and resolved. Then, the structure was very simple. I just cut leather in the same size, and punctured it. And that's all. So I recommend you to repair your broken HP-1.
And here are my orthodynamic headphones.
Welcome Kana! Great collection :).
What is the ortho on the top right of your photo? And is that Kawai definately an ortho too? (seems wikiphonia needs an update).