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Pileus :

The motor is raw water-cooled, and with time, it tends to scale up somewhat. My old Volvo MD17C is also raw water-cooled, and after about 20 years I had to strip it (for other reasons other than scaling), and it wasn't scaled too bad too bad, even though it had spent 15 years of its life in Houston and about 5 in Scotland. There was some scaling on the barrels and some pitting on the liners, but not much. cast iron is a forgiving material.
Raw water-cooled motors are cast iron construction everywhere where the (potentially saline) water is in contact with the engine cooling circuit.

If your motor was running OK before you replaced the pump, and still sounds OK, then you have not hurt it.

Take the wee cooling water temperature sender out of the engine....

...and clean it up a wee bit. Start the motor for a moment (only a moment) and water should gush out of the sender hole if your water pump is working fine. That should purge any air in there, if present.

Clean up the sender contacts also.

Try it again.

You can put a de-scaling liquid through the system by catching the hot exhaust coolant in a bucket and running it through the motor again. It might help. I have heard that a fair bit of scale can come out.

The sudden temperature alarm is very unlikely to be because of scaling though. Something else has done it. It is good you are checking the local temperatures with the heat sensor. That temperature of 138 F is not too hot. perhaps the sender has failed?

You could test it by making a makeshift earth from the the sensor body to the motor block, then drop the sensor in to a wee cup of boiling water to see how it responds. better still, is to heat it up slowly in a wee saucepan and check its performance with a thermometer (or temperature sensor). Set the sensor on something non-metallic that keeps the sensor off the bottom of the saucepan or you will get a false reading.

Let us know how you get on.

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