It's most likely that the points have corroded or worn (happens regularly on mine) Ideally, unscrew them, inspect them and clean/file/replace them. After standing idle, they get a layer of corrosion on them. Fine emery paper usually works a treat. Make sure you gap them correctly when refitting.
Next most likely culprits are the "condensor" if you have one, followed by an issue in the coil winding 9check with DVM)
It it really is your flywheel, your best bet is probably just to pick up another mower at a car boot and cannibalise it.
Borrow an inductive strobe lamp if you want to be sure that the ignition is working.
Dr Mark, August 2007