I beleave that in every alternator there is a cable that gets you peaks in a frecuency propotional to the RPM the probem may be know this proportional constant.

Optocouplers:
There are a lot. With AC or DC input. With transistor, triac or logic gate for the output.
If you use a transistor i.e. 4n27 you must swich on a led in the input of opto and you get a transistor for conecting as you want as output.
For filtering: Put a coil in serial with your circuit and a capacitor and an VDR in paralell. Change the 7805 for an lm323.
The values of thouse components are a function of your circuit.

Regards