I tried turning down an M14x1.5 sender to fit and it ripped in half when I tried to tighten it. Hard to imagine getting a half incher down to that size.
There is just a small electronic thermister, a hard piece of plastic with two electric leads, inside the hollow brass tube.
In fact, I have had a stock one twist in two with just slightly too much torque.
Your best option is to drill out the head and retap it or relocate it to a different spot. Lots of options for that.