Engine goes right to full throttle/advance

If you already have the head off I'd say check the valve faces/seats. If everthings ok, I'd replace the valve stem seals since you already have the head off. Looks like it went right through. :) Since head/exhaust gaskets come in pairs may as well replace seals on both sides if there's some miles and age on the motor. If the engine is relatively fresh, just put it back together.