Ouch! Looking at the dimensional differences between the various headsets, I was thinking at first that you could shim a set of ISO cups into the frame with three thicknesses of soda-can shim material (not ideal, but probably workable). You could also have any good bike shop reduce the fork crown from 26.7 to 26.4 to accept a standard fork-crown race. But the steerer threading seems to be a deal breaker. The thread count is the same as French, but the Austrian steerer is 1 mm larger, if I'm reading Sheldon Brown's crib sheet correctly. If you could find someone with a French die, you could maybe rethread the steerer, but that would reduce its thickness by a full mm--too much for safety, I would think.
Why am I even posting this? I'm no help. You need a replacement Austrian headset. I hope you find one.