Test the ignition coil first and replace the spark plug just for fun. Couple of possible internal problems; The magnet in the flywheel could have excessive rust and partially block the triggering of the magneto. The magneto may have shifted out of position. The magneto may be bad.
Since the magneto is under the flywheel, I recommend that an ONAN dealer do the job of replacing it. If you try it yourself, GET THE OVERHAUL MANUAL. When the flywheel comes off, a very tiny key tries its best to get lost. Be ready for it to fall out and under the unit. The key controls spark timing so it is necessary to have it.
When the flywheel is off, clean the sliprings and brushes using fine sandpaper (not emery paper) and electronic cleaner.
Have fun.
On Edit -- Using J-Ds serial number as an example, D is April and the 02 is the year of manufacture. The rest of the number is the actual serial number.