I would change the gear oil for the simple reason the jack shafts inside run one end in a bearing, and the other end in the cast aluminum housings. Ride it a few miles to get everything warm, and remove the bottom drain, and then the top vent. The easiest way to fill it is replace the bottom drain, and use a LARGE syringe (from any pharmacy) to measure, and fill thru the top vent screw.
Pull the Spark plug, and look into the end of it. It should be similar to a coffee color. Not black, and not white or light tan. If in doubt post up a Pict, and we can tell you what is going on.
It shouldn't need replacing by any means but I strongly suggest you buy another to keep as a spare. 2 T's foul plugs like leaves falling from trees.
Then like Kreps says... Check everything you can get a tool on. If you have any doubts Youtube is FULL of scooter related maintenance Vids, and someone here will always be happy to help.