1989 Yamaha 200HP 2 Stroke performance issues

[sgeo43nj]

Was running full out about 5000 RPM when engine, the engine started to lose power, then stalled. Had intermittent battery issues all season, so charging and jumping has been the norm all summer. Upon restart noticed a rougher idle, and rougher run, with a slight knocking. Pulled plugs to check for any sign of piston, cylinder damage - A blown piston would hammer the plug and present aluminum residue, so I've been told. Did a compression test and 5/6 cylinders tested normal between 110/120 PSI with the exception of the lower port cylinder. This cylinder tested 30PSI - Ran the boat today at 4800 RPM and it ran fine with a slight noticeable loss of power. I'm thinking it's not piston/cylinder - but perhaps a ring.
Any thoughts??? Clearly a rebuild on this engine will probably not happen, but if it's ring job, might be feasible. Any thoughts?

[99yam40]

never good to run a sick motor, all it does is hurt it more.
look inside the plug hole to see what you can see.
bore scope might let you see better.

or pull the head and see what you find.

have a friend that had twin DT150s on a boat that his son ran off shore one day.
came home saying there was a rattle in the lower unit of one of the motors.
tore it apart and found pieces of the piston and wrist pin down in the bottom of lower unit(exhaust).
hard to believe he did not notice a cylinder coming apart like that

[sgeo43nj]

Thanks 99Yam44, Going to try to look in the cylinder after removing the plug, borescope will probably give the best view...