While the sensor may be defective
, after so many years.....
I would first remove and clean it down to the bright shiny metal.
Years of suspended particles, (Calciums, Oxides, and other Minerals that may be dissolved in water)
have a way of building up on the sensors rendering the electrical properties inert.
( they won't work, or work intermittantly )
The way most coolant level sensors operate is by resistance to a current flowing thru them.
This resistance is low when coolant surrounds them completely, and all is well. (dash lights won't light up ).
When the level drops, there is no easy path for the current to flow thru the sensor,
this sets in motion, the Alarm signal. (Dash light lights behind a transparent wording of "LOW COOLANT")
So clean it first, then decide if it needs replacement.
You might save yourself some time and money.
Then you can follow the other "Ol timer" '* advice...
Yeah, I'm old too....