The 99-00 CA emissions cars have a different ECU, and slightly different code numbers. The emissions system has two cats and two O2 sensors. The PC on the CA cars reaaaaaaly chokes the power. The cast iron manifolds on all the 99-00 cars is heavy and doesn't flow as well as the '01+. It's a shame.
The '01+ non USDM OE manifold (Europe, Asian markets) is really something quite nice and *looks* aftermarket, but is hard to get and cost prohibitive because of shipping.
The key to your issue is this: your ECU is getting an errant reading that something is out of spec. You can replace the parts because they lose efficiency from age and use, but it really doesn't always cure the problem. This is why my PC went bad after a short period. You have to think about what makes a car push out more particulates (emissions), and that usually stems from unburnt fuel caused by a bad ignition. We all know plug wires are a common issue, but the coils need some attention too. As mentioned above, the '99-00 coils are a known weak spot, so as they degrade over time, you may not really notice it. Those $80 coils on Amazon I linked are great insurance that no matter what fix you choose to cure the p0421 code, you won't be destroying it like I did in a relatively short time.