I searched SD Johnson 16 hp and found a few You Tube videos. New to me. Interesting motor. I have 2 suggestions as to motors you might consider. First is the 1949-50 Mercury KF-7 Lighting. This Super 10 is actually closer to 15 hp. They are plentiful and not expensive. They can be some what of a challenge to work on as you have to devise substitutes for the special tools that were made for these motors but we have solutions for many of them. If you have the old impeller there is a member who can make you a replacement using the center from the old one. Used parts are obtainable, as well as some new, like gaskets, from several dealers. While these motor had 120 lbs or more compression when new, most have been well used. However the compression is 90 lbs. in this example I just uploaded a video of yesterday. After the repair part it shows the motor running. https://youtu.be/3aEQ773QzuM The second motor I would suggest is the 1928 or 1929 Elto Speedster. Even though you spin the flywheel (in the opposite direction you want it to run) to start them, these aren’t considered a “Row Boat” motor by some as they are not brass. Although rated at 7hp. its closer to 11 or 12. They have a very interesting ignition that will need some repair, and the coil will need to be rebuilt as the wires will be rotten if you find an example that has been resting for decades. The gas tank is the biggest issue with these because of the vibration. Look for cracks around the mounts. If they will let you, pour in some alcohol and check for leaks. I’d show you how to get the alcohol back out but too many links and everything here can disappear. They will all have dents. They don’t use an impeller but there are no seals in the gear case so it need some regular attention if your running them a lot. Your son may not want this much of a challenge but all the issues were over come. https://youtu.be/lJ6cXeWWi7g
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.