S2 filler neck won't work. Tried it once. The whole in the rear wing for filling is of different size mk3/mk4.

Just for reference, the S1 tank is the same as a mk3 XR3i tank or mk1 Orion 1.6i tank. The filler neck for a S1 can be taken also from the mk1 1.6i Orion, but not the XR3i mk3.