It depends on where it broke. If the head snapped off and you can get to the remainder of the bolt you might be able to get hold of it with a small set of needle nose pliers and screw it out. If you decide to try to use something like a screw extractor be careful not to damage the magneto cover when drilling. It's pricey to replace at about $200
If it broke off further down where you cannot get to the remains you are going to have to pull the magneto cover off the side of the engine so you can get to it. When you do that the remains of the bolt should just be sticking out where you can get pliers on it. Getting that cover off will require removing the water pump first. You will also need the gaskets and o-rings to put the magneto cover and water pump back on.
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