Engine overheated

[High Bidder]

This August my engine exhaust manifold and riser overheated on the right side of the engine.  Engine temperature was 170 degrees but the manifold became hot enough to melt the flapper and start the rubber coupling on fire.  The manifolds and risers are 3 years old and seem to be open as I can run water through them easily.  The outdrive impeller was new and is in the right direction.  Luckily it ran at an idle and stayed cool enough for me to get the ten miles back to the launch as water was coming in the exhaust and I have two bilge pumps but one seemed to keep me afloat.  Would this be caused by the outdrive pump, while looking good was not suppling enough water to the engine? Or was it a engine problem? This was a used engine I installed and it never seemed to idle correctly. I'm replacing it with a rebuilt long block but still don't know what caused this problem and don't want it to happen again. The only parts I didn't replace is the manifolds and risers. I installed new flappers, water pumps, thermostat and made sure the oil cooler is clear.