The last implementation of this kind of thing I have seen, was on a Go-Kart
The problem area turned out to be massive noise injected into the power supply from the engine and various inductive loads etc
The eventual cure was to power the circuit from a good quality dc-dc convertor and use a seperate amplifier for the pick-up with an optocoupler in the input of the meter board. All sorts had been tried to cure the problem, like ferrites, chokes etc etc, all had little effect because of the varying nature and harmonics in the noise. So the first thing I would do if I were you, is check the cleanliness of what you are going to power it from...
As you say, most of the modern bikes now have digital instrumentation, but unless it actually has a tacho that you are going to replace, you won't find anything to use that is straightforward in the ECU other than the odometer ticker, and that's driven off the gearbox and of no use for a tacho.
If you are lucky, perhaps your bike either has a tacho fitted, or can be as an optional upgrade, in which case you may be in luck