I just build a cheap tach adapter ie BHKZ switch unit for around $10 using
Radio Shack Parts. I converted my engine to COP (coil on plug) and change
to a all electronic ignition. Then I forgot about the tach and had to design this
unit. So if you want to try it out, PM me with your email address and I will send
you the schematic.

This should work on CDI units and Megasquirt.