Do not waste your money on a used top. It will probably be in worse condition than yours and eventually break AGAIN. The only real permanent solution is to 3D scan and refabricate out of titanium:
I don't have a convertible, but if I did:
Step 1. I'd have a local convertible specialist extract the broken part, (driver and passenger side).
Step 2. Take precise measurements (break out the digital caliper) and have an Autocad model generated for the part. If you don't know how to do this, find someone locally who does, maybe a shop that can 3D scan it and generate an Autocad file for you.
Step 3. Send the completed Autocad computer file to a firm like this to fabricate a new part out of titanium:
https://www.emachineshop.com/titanium/. Contact them, maybe they can also do the 3D scanning for you.
Step 4. Ask other forumers here if they were interested in a group buy to get the cost of producing the part down. If I was another random convertible owner on here, I would participate in the group buy even if my part was not broken yet, just to have them on hand for when they do, because it is a question of when not if. Heck, I'd probably swap them out before they broke, just so it's better than factory.
Step 5. Have local convertible specialist install new titanium parts.
Step 6. Never worry about part breaking down or corroding ever again; it's titanium. I chose titanium because the original part is made of aluminum. I bet Nissan (or their supplier who are building this roof for them) chose aluminum for this hinge because they don't want the heavy weight of stainless steel weighing down the assembly. Thus, as someone with an engineering background, I would like to warn you that if you had this part made of a heavier metal, it may throw the whole design out of balance and in the long term, cause other problems and break other components down the line. IMHO, titanium is the only option for a long term, permanent and perfect OEM+ fix.
As an outsider with a coupe, it seems to me that even if you are still under warranty, and Nissan replaces your roof you are just kicking the can down the road. By the time your new roof breaks (again) you will be out of warranty.
P.S. The original part looks like it's (die?)cast aluminum to me. There is no way to weld broken cast aluminum together and expect it to have anywhere close to the original tensile strength it originally had. Remember, the fracture happened there for a reason, as the fault line of the fracture represents the highest stress points in the part. Now you are putting a much weaker weld in the fault line, the same fault line that represents that highest stress datum along the part and expect it to hold. Any weld or repair to it will be useless and the weld will be ripped apart almost instantly as soon as the same stress forces are applied. It will just be a waste of your money in both welding it and the cost of labour to remove and reinstall it. The weld will be MUCH weaker than the original unbroken piece and even that still broke with the high stresses it has to endure.