is it a non-maintenance battery
Yeah I agree, follow this advice and you should be fine.The battery that come stock on the Ninja is a Yuasa YTX7A-BS, 12 volt 6 amp. Because of size constraints you will need to find a replacement battery (lead-acid or li-ion type) with the following dimensions or smaller. (Dimensions: 6" l x 3.4375" w x 3.75" h)
I found it rare for stores to stock this size battery. Even though it is a maintenance free battery, you will still have to fill the battery up with the provided acid and charge it initially, if the seller will not do it you you. So you may want to invest in a battery charger as well.
if the bike has sat up for a while and was not stored properly, you may want to have the carbs cleaned as well. If you were unable to verify the running condition of the the bike prior to purchase, I would go over the engine condition thoroughly to ensure proper operation.