A 5hp engine on that boat would make it fly. I'd highly suggest an electricc kicker though, say a 48lb thrust. But if you do go with a gas 5hp, Don't go with a side motormount. You'll really cavitate that boat. You'll want that kicker to be as DEAD center as possible. With a small electric you won't have the push a gas kicker has. I know my sled goes along pretty good with the original 4/5 horse that came with it. Can only imagiine what it'd do on a 9' outcast.
You'll also have to consider weight. You'll need an external gas tank if you want to run long distances. The weight of motor plus weight of gascan will definitely max out your boat. Problem with aan electric kickker is you don't have that much power if you use it as your main ssource of power.
I'd almost suggest you switch over to a pram vs pontoon boat. Alot of prams are light (not the 40lb light of a pontoon boat) and are a bit more forgiving with a motor. Just an option, and certain maaks of pram can hiit whitewater, or at least take you anywhere your outcast goes.
"Good angling ethic is not a monopoly practiced just by fly fisherman" - Bill McMillan