Every thing disappeared so I’ll try a it again in 2 parts- 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
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.