Thanks for the tip Glyn, just had a search and found this
http://www.beedspeed.com/revcounter.htm Base model looks very good value
But was hoping somebody had got a 916 onr to work, looks like i'm going to have to build a signal conditioning unit to pick up off the CDI output to the coil and get a nice digital pulse for the 916 tacho.
May just have to bolt my Stack on with its shift light and recall system, but its not in keeping with the project although it would give me a good idea how I should be gearing the bike which is going to be very important at the next two rounds