Not sure if anyone makes a jackplate for small motors and jon boats. From my bass boating days, you need to run a straight edge along the bottom of the boat extending beyond the transom. The cavitation plate on the engine above the prop should be setup approximately ¾ inch above the bottom of the boat. This would be a good starting point. If the motor has adjustment holes in the mounting bracket, raise the engine a notch at a time until cavitation starts and then back down a notch. Warning, running too high will cause cavitation and overheating because of lack of water to intake holes. The shaft length on the 9.9 hp may be too much for the transom of your 12‘ jon boat. You may need to modify/rework the transom to mount the motor high enough.