Running a Auto meter tach off of a Magneto

I have a question for the wizards. I have a magneto on my pro stock boat engine with an internal coil. Can I run an electronic type of tachometer (Autometer) off of the mag with the internal coil or do I need a mag with an external coil to make it work? There is only the kill wire coming out of the magneto. Would this wire give me a signal for the tach? :)eh:)
Any help would be appreciated.
Thanks in advance.
Paul

Running a Auto meter tach off of a Magneto