If you're just going to unset the variable, better to use a var.

As a general reply, there's no need to set the script variable, only accompany it, as such this should work:

alias quit { /quit $1- %script }

No reason to make it more complex than necessary.

edit: (directer ad original author)
as a side note, if you're looking to use the original quit in a script or on your own, use /!quit