Probably a good idea to change it if it has any oil in it.
A lot of new Vespa moped owners don't know about the oil in the rear hub so after sitting for 20 years it probaly needs oil.
You can buy 80-90 wt gear oil in a plastic nose cone squeeze container at most auto parts stores.
To change it remove the left side cover (belt side).
Look on the left rear hub near the Free Wheeling Button (black button thingy) for a screw or hex head bolt (if it is a bolt, don't confuse it with the two Top and bottom wheel mount bolts).
Anyway remove the screw or bolt. If oil runs out of the hole it is full. If you wish to change the oil, tilt the moped to the left and let it drain into a pan or wide mouth conatiner.
Refill with the new clean 80-90 weight gear oil until it runs out the hole (I think it take about 3-4ozs). Then replace the screw.
Check the old oil to see it there were any metal pieces in the oil.