Briggs and Stratton (and probably some others) offers a tiny tach that is about 1" x 3" and has an adhesive back. It also has a pickup wire that you just wrap at least 4 turns around one of the sparkplug wires. Of course, this is mainly counting sparks, so its probably only real accurate at about 3400-3600 rpm.
A lot of the commercial mowers use an oil pressure switch to ground their hour meters. Of course, you will have to consult a manual on your engine to find the oil pressure port. And come up with a pressure switch that grounds when it senses pressure. They wire fairly simply, a positive wire to the hour meter that is hot only when the switch is 'on' and a wire from the other tab on the meter to the oil pressure switch.
Some other commercial mowers only run the hour meter when the pto is on and the blades are turning. This does only count hours when you are actually doing work, but the downside is if the equipment is allowed to run with no load on it for long periods of time.