There are many options available, BTB, Scorpion, Powerflow, Nouvalari. Many opting to stick with the original, or want to replicate the original features via the BTB, which is also double skinned, both are quite heavy and only beneficial at high speeds with the Aerofoil characteristics. If top speed is not a concern or you want something lighter, sportier and cheaper, consider systems such as Nouvalari, who will design the system you want, varying the baffle content for a sportier sound and in the finish you want, single, double, twin or oval pipes, with or without a CAT. In these examples, the CAT was removed as legislation for all cars to be fitted with a CAT started in 1992.
Single, Double, Twin options.
The Z1 shares its manifold with similar engined cars of the era but the down pipe, cat and pipework to the rear silencer are one piece and unique to the Z1. If headers are fitted you would need fabrication work to fit them to the existing system or get a new centre section. You would also need to be mindful of the length of the headers, not just the width, so that the flange on the collector isn't the lowest point underneath the car. The oxygen sensor would need to be retained to keep the ECU happy. Headers will be louder than the factory manifold - the cast iron is very good at absorbing some of the harshness of the exhaust pulses - fairly thin walled stainless steel isn't. The Supersprint will bolt straight on as well as any LHD E30 325.-