Many people with overheat problems find that their exhaust elbows are partially blocked. Some have clogged core stacks - bits of impeller or engine zincs.

Did this engine always overheat?

If not, I would go through the raw water circuit (circuit is an appropriate term) from one end to the other looking for small issues that add up to a problem.

I have a friend that overhauled his engine only to find that his exhaust elbow was blocked (don't trust third world mechanics).

I cleaned the core stack of my old 4-108 in mild acid (vinegar) and was amazed at how much crap came out of it.

I have had bits of impeller and zinc in my new (6 Y.O.) engine that reduced water flow enough to increase engine temperature but not to overheat levels. I find that as zinc pencils sacrifice themselves pieces fall off. Every time I change the zinc I have to take off the end cap and clean it out. Half the zinc is still there but settled onto the bottom of the core stack.

Upping the size of the vented loop may solve the problem but if there are other issues it will be a temporary fix. You absolutely must have a vented loop if your engine sits below or near the waterline. Without it your problems will be far worse than overheating.

Good Luck.