Hi Wayne, I like to be able to help you on this, as I have some experience with electronic tacho's on cars etc..
( will have an ask on and a look on the car forum im on for you if you like), however there are stand alone units avalaible for mini motor bikes etc, that will probably take a pick up from the flywheel, is this the kind of thing you need though : http://www.abcmodelsport.net/p393895...CKET-SIZE.html