I have to disagree with 4pyros here.
I think that it will work, and it would work on a single cylinder lawn mower. One of the settings on the tacho is designed for a 2 cylinder, 4 stroke engine, that fires once per revolution. (Each cylinder fires every second revolution, so with 2 cylinders, one fires each revolution).
The magnetos on a lawnmower are on the crankshaft so there is a wasted spark system and each fires every revolution, so that will be right.
Also their website says "Modern DIS ignition applications are covered by the 2 cylinder setting" which is why that setting is there, rather than for 2 cylinder engines which I am fairly sure are rare in modern cars.
The bigger problem is that you will use about 1/4 of the range of the tacho because the engine only revs to 2400 rpm or so.