Those typ come with a 6 point 12mm hex bit (1/2" drive)
like this:
https://www.amazon.com/GEARWRENCH-80...s%2C163&sr=1-3
Soak the base and center area with a good penetrating
oil. Or make your own (50% Acetone, 50% Marvel Mystery
oil or ATF). Go eat lunch.
Use a breaker bar with the Hex bit and lean into it. Do not use an Impact.
Replacement nuts are avail thru Trigo Wheel. Just specify your thread pitch.
Trigo Wheels | Wheels For Your Cobra!
If the hex does slip (those pin nuts you posted look a bit worn) you can try a 1/2" Hex bit. (= 12.7MM) You may have to belt sand it a tad. Point being you don't want to crack the Pin nut as then you are going to have to Wiz wheel the Nut and stud off.