When engines are running well and then start having problems, it means something has changed. The inability to sustain full-throttle RPM without overheating usually means that something's worn out or is bent. One thing we noticed is that the bearings were not replaced when you replaced the piston, ring, and liner. That may be a source of difficulty. Corrosion products could also have caused excessive wear to the piston, ring, and liner, too.

Also, please check your fuel. If it has accumulated moisture in it, it will tend to make the engines run hot. Idle will also be problematic. If you have these problems with known, fresh fuel, then you'll have to disassemble the engines and inspect very carefully.