I used to replace plugs all the time as a preventative measure but, I have read on this site from an engine rebuilder that plugs do not wear out.............
That said, after a full-out rebuild of a twin cyl Kohler, all was good and it slowly developed signs of missing after a short while then hard stating, then NO starting.
I went ALL over that thing. Started replacing parts, carb service, adjust the timing, new ignition system from Kirk's, replace points....... on and on and on.
It turned out to be the plugs.
I guess you can't belive what folks say.