I've broken starter bolts off several times on saltwater machines. The fix is never easy and you should remove the exhaust manifold which will give you much more access. The best method I've come up with is to cut the ear off the starter with a diagonal cut. Then you can pull the ear off and grab what's left of the bolt to remove it. Usually what is left of the bolt comes out easily with vice grips. I've been tempted to pull the motor once or twice and probably ended up about a wash as far as time spent. For your case, first I'd remove the exhaust manifold, second I'd use vice grips and see if I can get the bolt out. If not, I'd grind the head off and cut the starter ear off. It's aluminum so it cuts easily and quickly with a 3" grinder with a fiber cutting wheel. It makes a mess with aluminum dust but you'll get it.
