Northern Tool & Equipment Co. sells a digital tach that works for a 2-cycle that only costs $36.99. I've never used one so I can't comment on their quality or quickness of responce. The catalog discription is slightly different from their web site discription. Be careful when ordering because they sell two different units; one for a 4-cycle that fires on every other revolution and one that works for a 2-cycle or 4-cycle that fires on every revolution.
Catalog discription: Crystal quartz accuracy and solid state electronics. Works as a tachometer when engine is running; displays total operating hours when engine is off. Easy-to-read LCD. Operates at 14F to 140F. Shielded 60.5" cable wraps around spark plug wire for pickup. Calibrated for 2-cycle engines that fire on every revolution. Also for use with 4-cycle that fires on every revolution.
The web site address is: http://www.northerntool.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=524744&prmenbr=6970
These tachs/hour meters seem more geared for logging run time hours for servicing so the tach feature may be less then desired. You may want to find a motorcycle junk yard and buy a tach from a 2-cycle motorcycle. Whichever way you go I hope this helps.
Lamborn's Miniature Engines