I've seen this done many times with a welder where the end of the broken stud is built up to where you can grab the built up weld or weld a nut to the buildup. It sounds scary, but unless the welder is an idiot with a large stick welder, it works like magic. The type of welder used is one of those small wire jobs that you can be precise with...I'm not knowledgeable on the tig, mig, etc. types by detail. And while it sounds odd to weld on a broken steel stud in an aluminum head, that is exactly the scenario that works the best. The two diverse metals in the weld heat cycle tend to actually help loosen the stud or bolt. And while it sounds scary to do this on aluminum, it is quite apparent that the head is a lot tougher than most think. If you think about it, there is combustion going on inside this head normally. Now...any idiot can use the best welder with the best intent and damage the head, but the many times I've seen this done on motorcycle and automotive heads...most of them aluminum...a good welder who knows his stuff doesn't even leave a mark on the head other than the soot from the burn which wipes off with a rag. I'd check around and talk to some good welders and/or mechanics who've used this method. Unfortunately if the space where the broken stud resides is so tight that the wire weld head cannot reach, then removal might be necessary.