Anyone know of a tachometer that will work with a 1978 85 hp johnson?

I've been looking at the ones in Cabelas, but cant justify spending $85 on one only to find my motor is too old for them...

Heres the harness.

http://cabelas.com/cabelas/en/...21320

Heres the gauge.

http://cabelas.com/cabelas/en/...01254