The 1.8 is one of a kind. It has a tailpipe lenghening arrangment where it leaves from the front of the back box and loops all the way back over it before ending under the back bumper. This must be to get pressure reflections right for mid range, which is very good for an engine of that size in a medium sized estate car. I got my last one via a mate in the trade, the retail was scary. Jug posted a link to reasonably priced unit on her a few months ago though.
in some cases only small mods are needed to make one exhasut fit the other. in the case of fitting a 1.6 exhasut to a 1.8 the only real difference is that the 1.6 is missing the middle box after the cat. this isnt a big issue, but i do think the length of the pipe is also different (1.6 looks shorter).
the later 1.6 has the cat on the manifold so that is even more different.