Check the ignition timing, should be 3mm before top dead centre. This is critical since incorrect timing will result in a weak or no spark and a weak spark may not fire under compression pressure. Over a period of time the ignition timing will retard due to wear of the plastic points cam follower. Open the point gap to advance the timing, close to retard. To check for a serviceable spark, regap a good plug to 3mm, attach to HT lead and earth to engine, spark should jump the gap when you pull the starter cord.
Alternative to all above, close normal plug gap down to 0.1 to 0.2 mm and install, mower will probably go OK. Reduced plug gap means lower HT voltage is required to fire and the square law applies, half the gap, quarter the voltage. Even if the spark is weak the smaller gap will allow it to start and once the engine is running there is plenty of output from the magneto.
