Has the motor ever been used in cold Northern waters or in salt water? Some motors sold in cold water areas had the restrictor mentioned before and it can plug up quite easily. Salt can corrode the area around the upper water tube grommet causing a massive internal leak which is hard to detect.
Was the old impeller missing any pieces of the vanes? Small pieces can plug the water passages and must be removed to prevent overheating. The powerhead will come off easily now in one piece so the water tube can be checked for any clogs in it. If you hook a drill up to the drive shaft, you can spin it to verify the pumps output. If everything checks out OK, pick up an infrared thermometer so you know for sure what temperature the motor is running at. If it’s still running too hot, I’d start to suspect a leaky head gasket.
Here’s a photo of the restrictor and it’s small water passage. If it was plugged you would have zero amount of water flow.
Attachments:
