[Arran]

You could try buying a generic 8'' cone and trying it, but measure the inside rim of the basket before you do. the issue might be that the spider, and voice coils are missing. I can't speak for what speakers Grunow used but I think they probably bought and rebranded speakers made by an outside manufacturer like Brand Z, or Crosley used to. A lot of Brand Z sets used Rola speakers, at least the bracket over the field coil looks like a Rola.
Regards
Arran