Had a Suzuki DF15 for 13 years that powered our 29' cat. It lived in a hole, was never flushed and was basically treated quite poorly. The only issue it ever had was that it used to suffer from spark plug
issues resulting in total or partial engine stoppages (how can an engine partially stop ? ..... One of the two plugs would fail). So, I'd try new spark plug
or two. For interest that engine may still be going strongly as it was sold with the boat 4 years ago and was running and starting flawlessly then.