You could probably get away with a 5 hp Mariner/Mercury 2 stroke on the back of your boat but it needs to be a long shaft model with a short pitch propeller.
I had a 4hp Mariner 2 stroke on the back of my 23 ft Windrush Wildfire (about 950 kg displacement) and it would scoot it along at 7 knots in smooth water.
However, it was a short shaft with a large prop pitch more suited to a tinnie so what would happen in a chop is the prop would come out of the water, the engine would rev it's tits off, lose suction of the cooling water and near burn out the cooling water impeller.
Also because of the propeller pitch, the engine was never able to reach optimal rpm and fuel efficiency.
The 4 and 5 hp Mariner/Mercury 2 stroke engines are single cylinder engines which are really simple and in reality are just updates of the Seagull Outboard Engine which are legendary for their longevity.
For the information of some of the younger folk here who through lack of detail or intentional disinformation in their historical education, I relate the following.
The Seagull Outboard Engine is an innovation that was driven by wartime demands.
In World WarII the British Expeditionary Forces were stranded on the shallow beach of Dunkirk. They had to be rescued.
The War Department went to the boffins (tech blokes) and said "Design and build us an engine that will take a 30 ft barge across the English Channel and back and do it in the next few weeks."
The result was the "British Seagull Outboard Engine", still going strong 76 years later.
, unless you use the outboard to turn. 
