You can now select 6 different overlays from the "Numbers, Flags, and Names" options. The overlay types are from my GoKart skin pack.
As a bonus, if you select Antarctica from the Country drop-down the flag will disappear.
If you are curious how to edit the paint shop definitions you should investigate the "assettocorsa\content\cars\gokart125s\ui\cm_paintshop.json" file.
There's also an explanation of the JSON file definition over here:
https://acstuff.ru/f/d/16-content-manager-paint-shop
Cheers,
Esotic