Disconnect the battery and charge it up. See if it goes dead soon or charge it and take it to an auto parts store/battery store. They will load test it for free at most places.
My advise is the biggest AGM battery that will fit. I have one that is 8 years old is still going. (On a carb'd atv) It gets put on a battery tender when it sits for any length of time.
You can search this forum for brands and sizes. I don't recommend the Walmart Everstart as it's +- posts are opposite of the OEM battery and sometimes hard to hook up. The last battery I purchased was from Fleet farm and I like it but I only has 2 years on it.