Upload & Sell: On
the mount should not change much between the versions.. you might even see if the mount from 24-70 works with this one.
Also, the ribbon can be reattached if you find someone willing to do the work.
Actually, the mounts on 24-70s (I'm not certain about 28-70s) are not interchangeable. They come in variable thicknesses, which Canon uses rather than shimming the mount to achieve proper backfocus distance.
I can't really tell from the picture, but it looks like the mount isn't damaged, rather the screws came out of the rear plastic barrel assembly. If the flexes aren't damaged (they just unplugged rather than tore) it's certainly repairable. If they're torn it just depends on whether they can find them (Canon doesn't stock them anymore).
But as someone said, worst case scenario you can sell it for decent money as a parts donor, especially if the front and rear elements are not scratched.