Replace the armature (ign coil) If perchance yours is an older point type ignition, put on the new style coil anyways and just nip the wire going to the points. The electronic ignition is by far a better spark than in the olden days. Bear in mind that a spark would have to be very weak in order to cause a no start. A person could check the flywheel key for shearing to cover all bases- especially if the either the flywheel nut (newer models) or starter clutch (older models) had been removed previously. Simply remove the flywheel nut or clutch, slide the washer off and inspect to see if the key is square w/ the keyway. Good luck.

Remove the plug wire.
Remove the plug.
Remove the shroud that covers the Flywheel.
Remove the two screws that hold the Coil to the engine block.
Use a small screw driver to find the magnet on the flywheel.
Use 400 grit sand paper to clean the magnet.
Use 400 grit sand paper to clean the coil.
Wipe the parts clean with a cloth as to leave no dust behind.

Take a piece of paper such as note book or typing and fold it in such a manner to get three layers.

Rotate the crank shaft so that the magnet is facing the Coil mounting tabs.
Set the coil on the tabs and let the magnet draw it to the flywheel tightly. This is called "Setting the air gap".
Install the screws that hold the coil and tighten them enough to hold the coil securely. Do not forget to install the ground wire with one of the screws.

Place the plug wire on the plug and place it where the threads are touching metal.

Spin the fly wheel by hand. You should now have a hot white to blue spark at the plug gap.

If it is a yellowish color. Look for other problems with the electrical system.