Are you going to attempt direct injection (injection directly into the cylinder) or are you squirting into the carburetor intake? If it's the latter and you just need to know engine RPM then it would be simpler to watch the AC frequency of the mower's alternator; to chop the 14V+ output to 5V you can use a linear regulator (see LM317 datasheet for example), a zener shunt, or a resistor divider.
Given that there are more gear teeth than alternator magnets you'd get a more accurate RPM reading with the gear teeth -- but I think that's overkill.
Also see: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1275359057