Hey, welcome.
Can't help you with the airbox sorry, I sold mine about 6 months ago
I still have heaps of other random parts lying around if you need though.
As for mods, what West said, just some general maintenence stuff to make sure the car doesnt die on the day.
S13's are great handling cars anyway, and thats more what the day is about, driver skill and handling.
There isn't enough room for power.
If you did want to do some quick things, front/rear strut braces I guess, and maybe a new set of tyres..?
It doesn't matter if you don't have any mods anyway, the aim of the day is to have fun, and there isn't any way you can't have fun at DECA.