All the panels are different on a Mk4 including the doors so it'll never look 'right'. If you are insistent on doing it you'll need front and rear panels, wings and bonnet. It still won't look like a real S1 as the doors are different.............but then you'll need the kit, opening rear 1/4 windows, dash, trim.............

Be alot easier and cheaper to buy a S1 in the first place.............sorry, not what you wanted to hear but just being honest !!