There is more than one size and type on most scooters including the AK 550.
Measure the diameter of the hole and I'll bet you can find an assortment on Amazon or the local discount auto parts store or most any motorcycle dealership parts counter.
Just have to look around. They are available.
If you want the exact same plastic rivet that you lost or broke then a Kymco dealer is the best source or search long and hard with the old one in hand to compare.