Using a 9v starter on a 12 volt system wont burn anything out if its just a motor (different if IC). Perfectly fine, the starter just gets more power than it expects. Anyway with a battery system, when the starter is running, it getting closer to 10v due to voltage drop durring high amperage consumption. Tolerances on starters are high and a ~30% increase in power wont cause any problems. Will probally use less Amperes than a 12V equilalent too. Good idea.

Dr. D