OMC lower unit
Nearly bought an S-2 with an OMC Saildrive 7 years ago. Did some research and found that while the engine was a 15 hp OMC/ Evinrude/Johnson with a big alternator, the lower unit was very prone to internal leg corrosion and there were no replacements available. Someone was marketing a replacement engine, a Honda 15 HP 4 stroke saildrive that would fit where the OMC unit was without modifications. They used to advertise in Latitude 38, so you might try a search. I have a sailing buddy with a 9.9 Johnson 4 stroke on his Oday 25 and it handles it just fine, in fact he can pretty much keep up with the 14hp diesel in my 28 footer. Good luck.