Tach for '86 3cyl 50hsp?

[outhere99]

I've done the searching, but now I'm more confused... What kind of tach do I need for my '86 2cycle 3 cylinder 50 hsp outboard (mercury)?

My throttle has a plug in the front, but I don't know if it's hooked up to the motor. I'm not against buying used, but want to make sure I get what should work on my engine.

Thanks, Jeff