so on a plane and under a load my engine was overheating. All hoses, etc are good, water pump is good, no blockage. Took the manifold off, all clogged up with junk. Cleaned it, flow is good, replace or reuse?
How did you clean it? Muratic acid? How does the area where the riser bolts on look? Is the metal there still thick, clean, and shiny? Or corroded and black looking?
If it was plugged with rust, etc, that used to be the metal inside the manifold. Yours is now much thinner inside. It is a gamble to run it. It is much wiser to replace it... but the choice is yours.
Replace both the manifold and the riser, you can't even get close to all the places that can plug up. Since you only mention cleaning the manifold, look at the riser. They are usually worse, especially where the water exits the riser.