Try a Tinytach... http://www.tinytachuk.com/

Digital rev counter and engine hour meter. Really easy to fit, one wire goes round the spark plug lead and the other goes to earth.

I have one on my car and its been to Lapland and back and there were no problems.

I'll post some pics of it is situ later.

I've got the wiring diagrams here somewhere for converting the GS tacho two two-cylinder. I tried to pick up the parts from maplin, but one of the bits was unavailable, they tried to flog me something for £19, when it should have been 15p or so.