Hi there,
I recently picked up a "machined aluminium" fork brace for my 73 750K from "Steve's Speed" on eBay.
He had a few for sale there, so he may still have some. I think he has an eBay store so you should be
able to search for the store itself.
Let me know if you can't find them and I'll send you the info directly.
Cheers,
Lou in Cold Lake, Alberta, Canada.