You need to go back with the same regulator, Lowe's or any other big box store will not have anything comprable.
Here is how to fix. Catch a little priming water first, then depressure and turn off the system/pump. First apply a little WD-40 or choice of lube to the backside. The threads go all the way through. Let them soak. Next carefully remove the 1/2" bolts. Be careful as these are the same rusty threads as broke on the adjustment screw. Also be aware that the assembly is now spring loaded and when you get to the last screw it may pop apart. Don't lose the spring or the washer. Once you get the regulator apart you should be able to easily remove the broke off adjustment screw with an ease out or drill it out and re-thread. Applying heat helps also. Examine the rubber diaphram of the regulator to make sure it's in good shape.
Here's a little wellman trick-take a hack-saw and cut notches into the plastic plunger, just enough to let a little water weep through. That will keep the pump from losing prime again unless you have a serious down-hole leak.
Apply a little anti-seize and re-assemble everything, making sure you back off on the adjustment screw to relase the tension in the spring. Once you've got everything tightened up you can tighten up the adjustment screw. On an MSE it needs to run at 60 psi with the pump running. This sets your jet pressure.
I always put a little anti-seize on the adjustment screws of jet pumps on install because something like this happens quite often. What happens is when the pump sits for a long while it loses a little prime and the regulator doesn't always allow the water in the tank to backflow into the pump to keep it liquid full. MSE's are quite suceptible to losing prime if they lose even a little bit of water.
It can be very difficult to remove the regulator from the pump. It has 1" cast-iron threads into the pump body and there is neither a good place to back-up the pump nor a good place to attach a wrench on the regulator. I have broken the cast-iron threads off the regulator, busted the pump, and I've even seen busted casings (PVC) and other things damaged. I would try this last. If nothing else just leave it alone, chances are it will work even with the adjustment screw seized up. Good luck.