For starters, the ZX3 was only ever fitted with the 2.0 liter DOHC (Double Overhead Cam) engine. The SOHC engine type you've quoted is not the correct one.
New catalysts are expensive due to the materials that go into making them. Finding a used one at the junkyard may be a better bet if cost is an issue as 1) you're ensured a correct fit and 2) cost will be considerably less than a new one.
Catalysts can be diffcult to remove if you have only a basic tool set. You need to remove the upstream 02 sensor (requires a special socket) and undo a large EGR tube nut. Rusty fasteners through out can also be problematic.