You could just remove the weapon if it's not in that level. In the clientside part of your 'go-kart' you could do
PHP Code:
//#CLIENTSIDE
function onPlayerEnters() {
if (player.level.name != "level.nw") {
this.destroy();
}
}
Then in the level it's supposed to be used you can do (serverside):
PHP Code:
function onPlayerEnters() {
player.addWeapon("go-kart");
}