I would pull the top part of the bolt with the bolt head on it out of the stem. That should give you access to the broken off part of the bolt and the wedge. If you have a punch that is long enough, stick it down through the bolt hole in the stem to the broken off part of the bolt and drive it and the wedge down with blows from a heavy hammer. If you don't have a punch that is long enough, find a piece of steel round stock the same diameter as the bolt that is long enough to reach, and use it as a punch. Once you have driven the wedge down, you should be able to pull the stem out.