Can you have a local machinist use the current steering wheel center and press-fit it into a new lathed out hub for you? If you werent in OZ, Id be able to do it for you quickly on a lathe at work.
Here is me turning a hub adapter for my 510
Unfortunately, I didnt finish it as other parts took priority over a steering wheel, but it probably only too me about an hour to do that. All that was left was parting it off, and drilling the bolt pattern in the top. Maybe 20min. worth of work... any competent machinist can hook you up.