Talk about having one foot in the grave!
I suspect that roverhi is right. They appear to be of a cast material and not really well done at that. My suggestion is that you bring a plate across from the top to capture the well for the gallows post and that you tie that into a plate mounted where the current
plate is. The gallows post would drop in as it does now but would be much stronger. I would think that 6061 T-6 aluminum
about 3/16 would do the job very nicely and it polishes up really bright. Because aluminum is soft it's easy to work with and you can do all of your own cutting and then take it to a local welder for assembly.
You can find what you might need at onlinemetal.