Just a thought but if you could get some measurements by looking at the drawbar receiver on your tractor (probably don't have to be real exact) and you could check a couple machine shops, if you have some close by. They may have a piece of scrap steel laying around just about the right size and could cut it/drill it as needed. I don't think that drawbars are made from any sort of special steel, like a hardened steel, etc.
edit: Going by memory I think the drawbar on the JD750 I had was about 2 1/2" wide x 1 1/2" thick, and maybe about 18" in length. Not sure what it is on the 790 I have now (haven't measured it) but it should be pretty close to that, or maybe even the same.