Just something I heard.....from C.A.R.B., if they don't know, than no one does, since they make the policies.
Call them and ask...
(800) 242-4450 California
(800) 322-9277 Nationwide
(626) 575-6858 Non-USA
http://www.arb.ca.gov/msprog/aftermkt/replace.htm
But simply replacing the ECU will not make a code go away. To use the 99--00 Si ECU, you need a conversion harness from OBD IIb to OBD I. Then, you will need to rewire anything like o2, vtec, I believe the alentator has a different number of wires, and a couple other small things.