if i were you, i'd drill the air box like a man on a mission and make it holier than a reverand, and buy a cheap ass air filter...then when its mucky and needs changing, its only a couple o quicd to change, where as if you buy a k&N, then thats it, 40£(or so) and it'll get dirtier and dirtier and you wont wanna fork out for a new one or a cleaning kit, and u'll loose performance.
40£ K&N or 4£ paper filter and big drill bit(1£)...hummm