Motor mounts are not rated for Horsepower but for weight. We had an 80's Nissan 2 stroke 9.8 engine that weighed around 65 lbs and the curious fact was that the 15hp model engine weighed only about 3 lbs more. Before you hang a 90+ lbs 4stroke engine in that mount I would ascertain its rating. Is it a fixed mount or is it one of those where you can raise or lower the engine? I'm not so much concerned about it coming of the backing plates but all the other components as after all it seems to be a 30+ year old mount. I notice you sail in a lake where any likely large wave would probably come from the wake of a passing boat and for that your mount may be OK but if you were coastal cruising I would say replace the mount for a new rated for the weight of the engine.