yeah, looks like it's pretty much brittle. Hard to do much good with that. Those forward two mounting screws are doing next to nothing, and would require reinforcement. I'd think you'd be better off trying to replace it with new.
Maybe check ebay?
If that's not an option... and I'm just "thinking out loud here...
gluing anything to plastic is sometimes hard to do, but not necessarily impossible. Eternabond two sided tape might be useful?....I'd look for some sheet metal...aluminum maybe...or some similar plastic sheets...and using screws and adhesive basically line the whole thing. Or maybe mask off some "molds" and use some epoxee to fill in the cracks and such....
If it's ABS, it's possible to get some ABS material (lego bricks are a source) and dissolve it in some MEK to make a slurry and basically chemical weld it to the existing. Figure a way to tape over the wholes with some release paper... kitchen parchment paper for example...and putty it from the inside for hopefully a smoother surface outside...
Maybe some aluminum angle bars bridging between the 4 attachment points holding the whole contraption down....
or
I have doubts though if any of this will work well since the whole thing is brittle, oxidized...and just pretty well shot.
Or Might be easier to just fab up a new box. Won't look the same but that shouldn't matter much
