I used to have a 3hp gamefisher, I don't remember the ratio for oil...maybe..100/1. But I do remember that if I mixed it too rich it would foul out the plugs, everytime.
The tube on the back should be your exhaust. It goes below the water line just to muffle it. There was 2 holes drilled halfway down the exhaust tube(1/8 inch holes), and I was told that was to eliminate some of the backpressure.