The CEN NT16 is actually a pretty good motor if it is your first. A new piston, con rod and liner will be cheaper than buying a second hand motor who's history might be worse than the motor it is replacing. CEN don't have the greatest customer service in the world but it might be worth giving them a call and seeing if they have the parts you need in stock. The diff cups are an easily broken part and everyone on this forum did the 'olive mod'. It is basically a 10mm compression fitting that you get from a plumbing shop. You superglue them to the diff cups and never break another one again! Compression fittings are also sometimes called olives, hence the title!. They will cost next to nothing for a pack of 10.
Other than that the car looks to be in great shape and well worth keeping. Have a search through this forum as it is a true goldmine of info about the Fun Factor. Of all the r/c cars and trucks I have had, the FF is one of the very few that I have kept. [8D]