My friend, you can begin from changing the manifold. There are two pre-cats and two main cats on the oem manifold. Just take an aftermarket one, or one of a 156 2.0ts euro2 and then, you can instal a metallic 200cpsi cat. This will improve the throtle response and the overall performance. Next step, a bmc cda intake with a 30cm pipe.Nice sound and also improved performance. Last but not least the ecu upgrade so that to take the max output of all the above..
If you want you can also use a straight pipe without silencers and also the rear silencer from the 3.2 engine. In this case, you will also have to live with more noise..
All the above, on my gt jts..YouTube - ‪Alfa Romeo Spider 1750 TBi - GT 2.0 JTS‬‏