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Took the cowl off, and the flywheel cover, and indeed there is a plugged hole on top of the block. It appears to use an allen head to unscrew it.

I don't know about Smartcraft, but when my new engine was installed and hooked to the old analog gauge package, the pressure gauge tube was neatly cut off and coiled inside the cowl, with clearly enough length to reach the "sensor" which as you explain is indeed probably just an adapter to match to the tube (which explains why the "kit" doesn't cost much - when a Mercury-brand aftermarket part is cheap, you know there's not much to it).

So I ordered the kit as it looks like this will work. Only possible unlikely issue is that the water pressure range is much different from the old engine's gauge calibration, which seems unlikely.

I got used to the water pressure gauge and it told me things, like fouled intakes, well before the high temp alarm went off. Do you really have to overheat the engine to tell you the flow is constricted, that can't be good in the long term.

I love the motor, every time I put it in idle I think it stalled because it's so quiet! And smooth. And I get at least 1-2 knots more speed at the equivalent RPM as the old engine. And yes displacement equals torque - gun this engine and the passengers in the rear of the boat fly off the transom into the drink :-)