As you suggested, I did adjust valve clearances but did not see much difference. I did ask an engineer here to take a look after all. After careful examination (He pointed my valve adjustment was all bad), we found out that one piston head was a lot lower than others. We are planning to take the engine out to pontoon for further examination and repair. So this is becoming a semi-overhaul.
Because cost is major concern to me now, the engineer will fix the main problem and take care of parts in very bad condition (injector rebuild and replace oil pipe, mixing elbow and thermostat).
I will do minor maintenance, cleaning and have the engine repainted (in red).
My heart is hurt for the extra expenses and time, I am learning a lot at least and happy to finally be able to clean my engine room.
If there is anything I should know before the engine rebuild, please let me know.