Replace the chain and sprockets.
-The stock chain is not "sealed" (meaning it doesn't have little o-rings that prevent dirt and grime to get within it that wear it out), the recommendation is to get a "sealed" or "o-ring" chain. You don't have to go with an o-ring, but it's a worthy purchase (o-ring chains cost more than non).
-The sprockets should also be replaced as your old sprockets are probably worn from the old chain.
-You still need to do the routine maintenance with any chain you choose to purchase. The o-ring just won't need as much adjustment as the non. But get in the habit of checking it to the recommended interval.
-The stock length is 122 links.