Since the nut is broken off, most of the tension has been relieved from the threads. Only the initial torque used to seat the stud is present, and that isn't very much. Mostly what is holding it is crud and galvanic corrosion. Soak it regularly with a high quality penetrating fluid, one that is designed to defeat corrosion. That is not WD-40. Give it a squirt a couple of times a day whenever you walk by it and let it soak for a few days so the fluid can work its way up in to the threads.
The best thing would be if you get a nut or another bolt or rod welded to it to allow you to more easily work it back and forth. If this is not possible, and if you have enough room to get access, drill it as dead-centre as you can. I recommend a good quality cutting oil which allows the drill bit to cut better, reduces the risk of breaking it, and reduces the heat because you do not want to overheat and harden the stud even more. Use left-hand drill bits preferably. If you get lucky, one of the drill bits might eventually spin it loose as you are drilling. If not, you needed to drill the hole anyway. Be mindful of the depth and to not drill to far and drill in to the head. Then try and work the stud loose with a good quality extractor. Don't try to just force it out with the easy-out, you'll break the extractor. Don't apply any lateral force to the extractor either or it will snap, and that is the last thing that you want to do as they are very hard and can't be drilled. Work it back and forth gently until you get a little movement. You can buy both left-hand and right-hand extractors so that you can work it in both directions. As soon as you get some movement stop. Don't get excited and go for it. Once you get a little movement, this means that you have broken the bond that the corrosion has on the threads. The fluid now has a better chance of penetrating further in to the threads. Soak it some more with the penetrating fluid while working it back and forth to help the fluid get in there. Then let it soak for a while again. Keep doing this. Hopefully you will get a little more movement each time until you can finally unscrew it.
If the above fails, progressively drill larger holes until you have gone to as big a diameter as possible without removing any of the internal threads in the head. Using a good quality cutting oil again and a tap of the correct size and thread pitch to match the internal threads in the head, carefully run the tap in to the hole to remove whatever material is remaining of the stud. Back the tap out often to clear out the metal chips so that the tap doesn't bind and break. Hopefully, you will be left with the original threads in the head undamaged.
If access is sufficient to allow me to properly centre and manipulate the drill to allow me to drill the hole where I want it, I usually go straight to the last method and drill and re-tap it.