I don't know of an electronic tach that will work on a 6-volt system. You might check with Sunpro, or one of the other electronic instrument manufacturers to see what's available. Some of the older farm tractors used a cable-driven tach like a speedometer, with a drive unit built into the back of the generator. You'd need a correct-sized generator pulley for an accurate reading, which might be pretty dificult to set up.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!