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If you mean you want it to charge while its running, put a GM 10 SI single wire alternator on it. That's if it has a 12V system. There are also 6V one wire alternators. Ebay has many to choose from.
If you are talking about when it is parked outside, HF sells solar chargers that hook to the battery. I have two of them keeping my two trailer batteries charged. If inside, HF also sells plug in wall cube trickle chargers. I use them in winter on car batteries stored in the garage. Solar chargers are about $15, cube trickle chargers about $10.