You can work from the schematic you posted- the missing metal from the top is the only important part and as shown, is for the little shiny trim piece above the DIN panel and a mid mounting point for the DIN cover screws. And as you suggested the radio mounts screw to the inside metal area you can also see in the schematic.
You could attach some metal (maybe even a piece of stereo system hanger metal that comes with the kits?) across that front area and provide holes for mounting the trim strip, and maybe even provide for the two mid mounting screws with obviously some cutting/notching/dry fitting and other monkeying to get it all lined up and looking right. My suggestion would be to use 1-5 minute epoxy with clothes pins or other holding method to fasten your new front metal piece (s) since screws/fasteners will be difficult and steal space.
With the screws in the top and friction clips in the bottom the DIN panel (black area in schemo pic) should mount fine as well with maybe a little prodding...for reference, I did a "custom" wood DIN pocket and other additions and fixes to my DIN panel with broken mounts in my NA (not trashed like yours though) and it really isn't too hard to get it to go back together OK since there's some friction fits involved that do some holding for you as well with the shifter cover and all.
I'd just try to make it a fun fiddle-around project with priority of getting everything to fit and fasten right, and not worry about how you did it or how it looks inside- just get the system to mount correctly and the DIN panel to go back in and stay tight and you're golden.
Sorry if I make it sound overly simple but it is one of my strong suits- fix and improve.
It can be done...
