The green wire should be switched power, the case should have a ground lug. Describe the two white wires. Are they looped on the back of the tach? This sounds like an RVI tach and if so, any 4-cylinder RVI tach will work fine in its place. You should find "RVI" printed somewhere on the gauge face.
Keep in mind that if you remove an RVI tach from the car, you have to connect the remaining white wires behind the dash (to each other... and insulate them) or the car will not run. Those white wires carry power from the ignition switch to the ignition coil.
You can also replace the RVI tach by fitting an RVC tach (voltage sensing) in its place. To do this, connect the white wires behind the dash as mentioned above and pull a new sensing wire from the RVI tach to the low (point's side) of the coil. As before, green wire to switched 12V power and case grounded.