and still you have not answered my question about if the motor is running to heat things up to open the stat while you are flushing with fresh water.
or maybe that yes was confirming you do run the motor while flushing
compression is easy to check, and a carb clean & pulling stat and flushing the cooling system with something like Ridlym marine descaler is not that hard either
I guess you have a leak even with a new OEM oring.
are you sure the surfaces are not pitted or damaged.
or maybe the oring got damaged during installation.
Sorry, I have no idea where you could find an oversized oring
so you tried doing the procedure after doing the oil changes and that did not reset the hours for the alarm?
did you try contacting Suzuki to see what you could try?
I would think a owners manual or service manual for that motor would tell you how to reset the timer after an oil change.
Just like they do for automobiles.