Single cylinder tacho???

Hi all, I'm building a generator/ compressor combo and I am using a single cylinder stationary engine of about 4HP to run it. I would like to hook up a tacho to it but where do I find one for a single cylinder engine?
That's my question now here are the details: We occasionally need compressed air and/or a 12volt generator on trips. I have used an air conditioning compressor as an air compressor for many years on my old truck but on my new one I don't have diff locks ect so it's use is more limited and I would prefer to have something that is not in the engine bay. I have an electric compressor but nothing pumps like the air con pumps. The 12 volt generator will simply be a Bosch altenator and will be used for those times when a drive of an hour or two is needed to top up the batteries or some one gets caught with a flat battery but won't be for direct jump starting. The tacho would help optimise setting the revs for each application. The whole lot sits on two small air cylinders under the motor ect. It is reasonable in size measuring L500mm X W250 X H 450. The idea behind this project is to use it as a back up unit. It will cost me virtually nothing as I have a great little 4 stroke motor, altenators and compressors, most of the gauges and bits 'n' pieces I need and a lathe for the machining so it's a project come something to tinker with in the garrage. I have also made a very good (read quiet) muffler for it. I have geared the altenator up about 5 to 1 and the comp down about 3 to 1. I haven't run it yet as I still need another day or 2 to finish machining up the pullies. Any CONSTUCTIVE ideas would be most welcome. Cheers.