There are 22 foot pontoons out there with motors in the 115 range. In my view there are many things considered when the manufacturer rates a pontoon for a certain size motor. This is just my thoughts and not based on any design regulations. The pontoon has to handle the weight of the motor along with the rest of the people and gear. The boat needs to be balanced. The transom structure has to handle the horsepower and transfer that power to the boat in all conditions including hard turns without becoming unstable. The boat has to perform, steer, and otherwise handle the horsepower safely. I'm sure there's more but that's a few things to consider.
If you're into the math it takes, you can figure how much weight a pontoon can float fully submerged. Take the volume of the pontoons and consider that a cubic foot of water weighs about 64 pounds. So if you have vessel with 50 cubic feet of volume it would take 3,200 pounds to submerse it. Then as a guide you want to cut it in half so the pontoon floats half out of the water. But just because you can float the heavier motor doesn't mean the boat can handle the horsepower.
Common sense and safe practices rates very high in my book on boaters safety.
