Custom car bodies, decals, trails, and boosts not appear in the locker room and during gameplay on UEFN maps in Fortnite.
Some steps for you, if you want:
1- Enter in a creative map, preferably UEFN.
2- Enter in a Changing Booth and access your locker.
3- Go to your cars, If and they won’t appear; you can only change cars by saving your presets in the lobby before entering a map