Did you find it broken compliments of a previous owner, or did you snap it yourself? If it broke during tightening, you might be amazed by how easily the remains might unscrew out of the head using a simple screw extractor, or even cutting a slot for a straight screwdriver bit with a Dremel.
If the threads are siezed and it broke when you were attempting to loosen it, then you're probably right that it will need to be drilled out. Whether or not that demands head removal depends on how careful/patient you are. If it were mine, I'd pack the area off with rags, grease, etc and give it a go right there in place. Nothing ventured, nothing gained.....if you can't get it done in place, then pull the head.