The catalyst must run at a very high internal temperature to work. If you have a large enough catalyst, you may not be able to keep it hot enough at idle to work well. In California, aftermarket catalysts have to be certified. Practically, if the catalyst works well enough for you to pass the test you can use anything you want. There is no easy way for the inspector to tell if the catalyst you have is certified or not.
Most thermal reactors are more restrictive than a catalyst anyway.
A MkI NA motor can use a MkI SC catalyst and exhaust sytem with no impact on emissions. You will have to swap O2 sensors since some cars use a 2 wire sensor while later models and the SC car use a 4 wire O2 sensor with a heater element. The exhaust manifolds are NOT compatible due to different EGR systems. Ignition timing - Retarding ignition timing will usually reduce emmisions somewhat. Also, if the idle timing is checked, you may fail for having it set wrong.