the s1 has a 6 bolt flywheel and the series 2 has an 8 bolt, the thermostate is located at the front of the intake manifold on the s2 and the s1 is located the arse end of the engine, hence you'll see the hose running back behind. Fuel rail was updated in s2. the radiator has a cap on it, whereas the s1 used the overflow bottle for the main system if you know what i mean. on the very early s1's the zort manifold's flowed better because of shape, kind of a premature extractor like, and thats all i can think of for now
but if your thinking about changing motors between series, ive heard it's way to much hassel, specailly the flywheel,
oh also the s2 has an inspection cover on the tranny whereas the s1 did not