all mk2 cars have the cat built into the midpipe...
so a replacement midpipe without the cat is what you need.
this will fit
http://www.mx5parts.co.uk/stainless-sec ... -1250.html but you will have an o2 sensor hole you wont be using.
failing that you might want to try your local custom exhaust shop and get them to cut the cat out and weld in a straight pipe.... i paid £175 for mine 5 months ago and it does add a good bit of torque although the downside is that it totally changes the note of the exhaust

and not in a good way.... think chavtastic