Yes, the water pump belt breaking will stop the tach from working. I found that one out the hard way one time when I was young, dumb, and did not know any better in the 80s and I had a "77". I just went hmm, the tach stopped working, I guess I will check it when I get back, and kept riding. I did not get too far before it seized. It was not a bad siezure thankfully but I learn that that tach was the warning. I don't remember anything about how much work it was to change the belt but I am thinking it was not too bad.