I am no specialist, but like Action said, I would try to fix the old one. You have to take it off anyways to install the new one.
IMO, for the money, the aftermarket one should have no gaskets showing around the windows to match the hard doors for a clean appearance, it makes it look like a cheap and lazy design. Also the two top hinges should be internal or different design and not show.
Not sure if it is applicable to your truck, there are some videos on YouTube made by Predator where they install a similar aftermarket slantback and they have to fabricate some brackets and make some mods because it didn't fit out of the box.
And there is some probability/ risk that you may not like the final look and fit when seeing it in person.
It sounds like your real motivator/ pain point is having a window/ windows in the back and a sunroof, and the rust may just be a justification to get there?
I have not seen your truck, but how about finding a local prototype metal fabrication shop (somewhere within driving distance so you can be there in person to get every detail ironed out from the beginning and watch their progress closely in case there was any miscommunication - lots of flexibility and options - I have some customers that do 3D printing in copper for some rocket parts even for production, not just prototypes because it costs less than making molds - not that it is applicable to this case, but my point is that protorype shops have a lot of tools that may be a better fit) to add a window/ windows (without showing the gasket, maybe emboss the edge and fit it under the window, which would be attached from the outside, to fit flush with the sponson and/ or hatch surface, you could even add electric tilt if you really wanted, and would still cost less and look better) to your existing setup and have them make you the top part duplicate to fit but from aluminum and with the cut for the sunroof instead.
