a lot more than you think,
parts different are,
loom,
dash,
doors,
lockset,
kit,
wings
glass colour
mirrors
rear panel
engine block
gearbox
front anti roll bar setup
rear brake compensator setup
smaller clutch/flywheel
all dash brackets,
bulkhead sound deadening,
interior
abc pillar trims
bootlids different as mk4 is recessed round the lock
bonnet obviously,scuttle panels different
brake crossover bar on s1 are smaller
brake servo/master

kjetronic setup is different too only 1 sensor on inlet manifold,
inlet manifold splits on s1
no abs
washer bottle
header tank
electronic dipstick on s1
floor numbers in a different position

loads of other differences mate