After a very long and disappointing search for parts I think at last I've had some success. A Tempo-freunde in Germany has managed to source a pile of left over parts from the Arbeit Museum in Hamburg, which they have kindly sold to me for a nominal price. Some parts, such as the chassis tube, are in pretty poor shape but they will form a template to replicate a replacement.
Three wheel hubs
Brake drums (including full wheel hubs, suspension, springs, etc)
This spare tube chassis was donated by the museum. It's in rough condition but will be a template for a replacement.
The steering head pivot is a critic part that would be difficult to replicate. The cross members will all be replaced.
The rear cross member has some rust holes and repaired damage. I'm not sure if it is salvageable, but we'll see.
The heavy parts ready for shipping. To fit it into the box the chassis has been cut into three parts.
Ready to go!