It’s an easy check to remove the thermostat cover and thermostat, then test run the motor briefly. At least you could verify that there is good flow to that point. The gearcase can be tested separately by immersing the gearcase in a bucket of water to just cover the impeller housing, then spinning the driveshaft with a drill and observing the flow from the impeller housing. It’s just a matter of systematically eliminating possibilities of a blockage or impedance of the water flow.
Fleetwin has the right advice. “I guess I would start at the beginning.”
Recently, I worked on a 1963 Johnson 18 hp that had two problems “at the beginning”. The impeller was pulverized to the point that the exit chamber in the impeller housing was 100% clogged with tiny pieces of impeller. In addition, the water tube grommet was folded back into the impeller housing blocking off a lot of the flow.
Improvise-Adapt-Overcome