84 California II tachometer failure.

[Peter949]

After 38 years of use, and 145,000 kilometres my tachometer reads about 1,500 RPM too high. The needle sometimes sticks at about 2000 indicated after I turn off the engine.     
I was hoping to find a cheap digital tach that will work on my 84 Cali II. At times I shift at very high revs with the steep hills near my farm, and I simply like to see how fast my old engine is spinning.
If anyone can suggest a digital tach that will actually work, I would greatly appreciate your advice!