I would strongly suspect that the cut-in a bypassing of the safety switches has much to do with the tachometer not working.
I just acquired a Kubota G1800 (diesel) lawn tractor and the previous owner did the same thing. Bypassed everything. The temp and tach are non-op. And, if you disconnect the control module (as you're supposed to do when charging the battery), the engine will not shut off
. I'm proposing that just by bypassing one or two things causes a ripple effect in other areas of your machine.