My 9.8hp 2 stroke Tohatsu is working great on my Cal T/4. I think it is at the top end of the size of motors I would put on this 24' boat. It gets me in and out of Pearl Harbor with ease. My friend has a 24' McGregor that has a 4hp Yamaha 4 stroke and it also works well coming in and out of Pearl Harbor. I would think that a 5.5 would do well on the Cal 20. I would see if it is a 20" or 25" shaft because the 25" shaft will stay in the water better that the 20" if you get into any large swells. I have the 20" shaft with a lift up motor mount. I can set it down on the lowest position and do fine but my friend's has a fixed mount and his motor will come up out of the water some times.