There are sooo many threads on this subject that if you so a search on it it will make your head spin!
With all due respect to you, a search can give far more detailed in information than I will here.
In short, yes it can be used as a charging source.
1. It must be one of the (1 in 100 chance yours is one) properly wound magnetos; most are not properly wound and will not put out enough power to charge or even power a light for that matter.
2. You will need a 1/2 wave rectifier (4 watt diode).
3. In addition to #2, it is best to have a charge controller too.
A search on this subject will enlighten you beyond your wildest dreams.
Once again, I mean no disrespect in any way, shape or form towards you.