If you can show proof that the units are sold legally in California and
possess an EO number, (EO stands for Executive Order - this is the number
given to the product by the California Air Resources Board certifying that
it is California emissions legal) then you shouldn't have a problem. Most
manufactures of aftermarket exhaust or emission components will stamp this
number on the actual unit or they will print in the units manual, catalog or
some other paperwork which pertains directly to the component in question.
You will have to present this number to the smog station where the car is
being tested. You'll absolutely need to keep the oxygen sensor but the rest
of the exhaust system has to be California approved as well.