timing...

When you say you replaced the thermostat, do you mean the "coolant thermostat" that is on the upper radiator hose, or do you mean the "choke thermostat" that is a bi-metal spring in the exhaust manifold with a rod to the top of the carburetor? That later would affect your cold starting. You mentioned in another post that the choke plate just flopped open and I suggested changing the choke thermostat since sounds like its spring was broken.

I think what your mechanic did in bypassing the vacuum thermal switch in the radiator is fairly common. That is an emissions device that fails and I think is hard to get replacements and is not essential. I have never had a newer slant with that part.

The term for the "top of the engine" is the "valve cover" (blue in photo).