Plenty of compression is not anything to do with it.The plug must be faulty if you get 'a good belt'
If it still won't start after changing plug, it is the ignition unit.
Latest models have electronic units, while the earlier ones have points. Remove flywheel this exposes the cover of the ignition unit. Remove, and then remove points-dismantle and clean with emery. Re-set at 30 thousand. Rebuild.
mowerman
October 2008