Its “Rejetting” I hope It means that replacing or modifying some of these jets so as to supply gasoline differently than stock, usually more gasoline than stock.
Yes. It’s likely that the sprockets are worn, and a new chain will make the wear out of the socket faster than expected so its recommended to change it.
Chaning the Oil- There are different versions for this question one is that you can change it every 3 months and the other is that change every 3000 or some say 6000miles