As awful as that looks, it's possible the yoke may still be sound. Some covers were made of a miraculously cheesy plastic that self-destructs. I haven't seen one quite that bad before, but I have seen covers that were warped all to heck, with pieces falling off, etc.
I would withhold judgment for the time being. If the yoke turns out to be OK and you finish the remainder of the project, you can fashion a substitute cover later on.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html