At this stage it is probably not the magneto then. Notice that I said at this stage. THe next thing to consider what type of rev counter you have is it electronic or manual, with a cable comong out the back and going down to a drive gear. If the rev counter is electronic or electric, where does it get it's signal sorce ? This will mean tracing it back through the wiring. So the easy way to do this is to:

1 check what kind of rev counter it is

2 If electronic see if the maneto has an electronic pickup mounted on the side of the magneto

3 If it does not have an electronic pickup then you will have to search through the wiring to see where the signal sorce is.