Advice needed on outboard engine

[ansan12001]

First off, this is actually a Nissan Marine 4 stroke, model NSF6BS. I used this engine on a 15 foot or so Valco, mainly because I got the motor for 300.00 and its light. I dont install the motor on the boat when transporting because the transom is rather light duty it seems. I use the boat in local reservoirs and San Diego Harbor. She gets the job done on very little fuel.
The motor is never going to get me anywhere quickly I know that, but I was wondering if I can get more water displacement with a different prop (see pic)? Not concerned about acceleration just trying to get any long distance mph increase.
I?m trying to take care of the engine with maintenance. I plan to change out the rubber lines and fuel filter bc lines are brittle. I am posting several pictures. The picture of the carb shows a open port near the top of carb. Is it supposed to be this way?
When I removed the cowling, I noticed a thin wire with a stopper on the end. Looks like it broke off somewhere but I have no idea where. I found it on left side of engine and tried to mock it up in the photo. Noteworthy, the throttle doesn?t snap back to idle on its own so Im wondering if the cable is for the throttle return?
Im probably going to keep the boat for a while still and thought I can service the engine and leave in storage since I already took the appropriate actions to run out the fuel in the carb at last use.
Id also like to hear suggestions on a bigger engine that doesnt add a ton of weight since I dont leave the motor on transom during transport, I carry it from the back of truck to boat before launching. If I was planning to move into a large boat in a few years, would 20 HP be enough for a 20 foot boat for near shore fishing (less then a mile out I from San Diego harbor). A few companies have a newer model 20hp outboard for about 100 pounds so I would definitely like to stick to that weight limit. Sorry for the all questions but any advice appreciated! The plans to buy another engine really is just a dream ATM but I need to have a goal and budget in mind!
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[ansan12001]

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[G-forceJunkie]

Small motors like that the prop is sized for the motor, doesn't matter what its on so no, your not going to see any gains with a different prop The hand throttle on most outboards don't snap closed, otherwise you would have to hold tension it all the time. Thats also why wearing the kill switch lanyard is important so it doesnt run away if you get thrown out. The only open port on a carb I can think of would be the bowl vent but they usually have a short hose on them, if the float sticks open it allows the fuel to dump out