Forget modding or replacing the airbox, no point and will end up giving you more greif in the long run with carb icing and power loss due to it sucking in only warm air from the engine bay. At most fit a K&N pannel filter to replace the stock filter

If you want REAL benefits, fit a full MK2 GTI 8v exhaust system including the manifold and downpipe, makes the engine rev better. If you don't want to replace the stock manifold, you can fit an 1.8 Driver downpipe which fits the 1.6 manifold and has a bigger bore on the other end allowing you to fit the GTI system from the front silencer back.
Then get an 8v GTI cam which should give you a nice boost, then finish it off with a webber carb re-jetted on a rolling road to makle the most out of all the mods
