The good hooks are L-shaped and look like this:
And the broken ones looks like this:
In the same picture you see the replacements I've manufactured.
First, peel off 4 layers of fabric at the corners and undo the 40 yrs old screws. Then drill holes in the new hooks, first the big ones and then the small thru-ones (learned by doing it the other way around and breaking the hook):
And then screw them back in: