OK, so you replaced the SS plate, the impeller and the aluminum hsg, correct? That complete water pump kit is very expensive! Did the motor run hot/overheat before the water pump work? Will the engine pump/cool OK if run low in the water with the joint submerged? I guess this is a tough way to judge the pumping because the outlets are under water!
As you know, from owning other lightwins, these engines run warm/hot on the exhaust cover because it is not water cooled. The head tends to run a little hotter than other OMC models as well. The best way to evaluate the cooling performance is to take temp readings on the right side of the block on the vertical water passage that runs down and out of the powerhead.
I am not that familiar with the components of the folding engines, and how the water tube is connected/sealed. I do not that sometimes the water tubes come in contact with the driveshaft and gets rubbed through causing a leak. But, on your engine, the upper water tube is fairly stationary, and not affected by gearcase removal/installation, so I am guessing a bent/pinched leaky water tube is not the issue.
Is this a fresh or salt water engine? Some of the internal passages are pretty narrow, and get plugged with salt easily. This sounds like a real nice engine, so we look forward to resolving the issue to avoid overheat damage.

Post pictures if that will help…