Wow, if you can cast brass, you're all set up, and you may find all of a sudden a whole lot of new friends! I saw another case where a bezel was replaced by sawing one out of wood and carefully shaping it with sandpaper. A wood with a swirly grain was chosen to mimic the original plastic. Of course it could be painted if wanted. Just some ideas.
Seems odd to have a rear cover fastened with nails which would limit access to replace tubes, etc. That could have been something an owner added. I have made rear covers for radios by jigsawing out of 1/8" masonite, like this one:
