In the first place, it is an Evinrude, not Johnson. It is a 1963. And it is 5hp. Now the rest of the story: OMC come out with those 5hp motors to compete with the cheap Eskas being sold by Sears and others. They figures people would buy a quality motor if the price was low. Didn't work, people kept buying cheap Eskas. Revolving charge accounts y'know.
As for the motor, it was a 6hp powerhead with a tiny carburetor knocking the power down to 5hp. Keeping the price down, it had what basically was the 4hp Yachtwin lower unit, with no gearshift. It also had no thermostat, but one could be installed. Put a 6hp carb on it?? Sure you could, but you wouldn't get 6hp performance because of the lower unit, which by the way, you would be stressing.
5hp propellers are about as rare as chicken fangs, if you have one.
