Generally a weak magnet shows an ok idle, low power in the upper revs if you can get it there. Won't take throttle.
Easy to test, with a fully charged battery unplug the reg/rec. If it runs ok that way the magnet is weak. It was an easy fix when Radio Shack had stores everywhere. Might try online. Radio shack item # 64-1895, two pack for about $3. These are a 3/16 inch.4.5 mm Rare earth magnet. Just glue one on top of the magnet in the rotor. Any good epoxy that's heat resistant should work.
Leo