The "magneto" is the combination of the flywheel with its magnets and the coils of wire behing the flywheel. That is it. There is nothing to "recharge". It is extremely unlikely that the magnets have weakened. I have never heard of that happening. And if it did, then ALL lights and the spark itself would be weakened, not just one light. It is possible that one or more of the coils has burned out, but that is also very unlikely. The coils are for various things. One will be for supplying power for the ignition. Another will be for the lights. Another may be for the tail light and/or brake light and/or horn or charging a battery if applicable.
The most likely things to cause dim lights are:
1 wrong bulb(s)
2 incorrect wiring. Someone mave have connected wires where they should not be connected or unconnected something that should be connected.
3 Poor wire connections due to corrosion.
4 switches making poor contact.
5 partially shorted wires (pinched somewhere). This would tend to be a flickering lights situation.