Where is the low flow? If it is on the raw water side, then the thermostat is not the problem. The thermostat is on the fresh water side of the system.
If you are not getting good raw water flow, that is a raw water issue:strainer, raw water pump, heat exchanger, exhaust elbow, plugged hose, plugged inlet seacock. When you took the cover off the water pump, did you crack the seacock a little to be sure you have free water flow? If you sucked a plastic bag up the inlet it will shut off water flow and cause overheating. You can probably guess how I know this. If it is a low raw water flow problem, for now confine your troubleshooting to the raw water side of the system.
I also have a 1985 westerbeke. If you have never cleaned out the heat exchanger you're way, way overdue. Frequent replacement of the pencil zinc is a good idea.