Use
Kroil to loosen that broken off bolt. Let it soak in overnight and then apply some heat to the tiller arm, which should be steel, so a propane torch won't hurt it (but keep it away from motor cowl!). Best penetrating oil I've ever seen! Pratt & Whitney used to use it on seized bolts that saw 1200-1400F, temperatures that no anti-seize compound will survive. A friend of mine that has a side business overhauling turbochargers also uses it and swears by it. Good hardware stores carry it, but you can also order it direct or from Eastwood, the outfit that sells car restoration stuff.