Hi Erick,
Welcome to the group. The first S2 27 I ever saw was in New Hampshire. It was being rebuilt near Newmarket about 4 years ago. I couldn't make a deal with that owner and started looking around closer to home.
The Yanmar 1GM10 can push my boat at about 5 knots. If that's enough for your river currents it has to be cheaper to replace the starter & alternator than buy a new, more powerful outboard and the mounting brackets, plus installation that you'll need.
In my humble opinion,
Bob
