A ‘keeper’ is just a bar of iron or steel placed across the magnets poles to help the magnet retain its power when not being used. Small horseshoe magnets from the hardware store usually have a keeper on them. On several NOS flywheels I picked up from an old dealers stash, a bolt had been used as a keeper.

The weaker the magnetic field is, the weaker the spark will be. One Johnson 300 I have would not run well until the flywheel magnets were recharged. Now it starts and runs like new.