Being that you have no hex portion to grip onto, you're kind of screwed. First off, how to the hex head snap off? It looks like someone over-tightened it based upon the wear on the transmission case.
You can also theoretically snap off the electrical connector and drill out or use an extractor on the threaded portion; however, if there is any debris, its going to be falling into the transmission which is not good. Or a little more technical; snap off the electrical connector, weld or jb weld another bolt to the threaded portion and use the new bolt to extract the threaded portion that remains. But this way is more incumbents of how seized that threaded portion may be; you might get luck and it may just pop out, or it just might be totally seized and needs to be drilled and snapped out.
Personally, I would just leave it as is. Depending on the space available, you or a legit mechanic would have to drop the transmission to be able to do it the right way.
Either way, let us know what you do.