It does away with all that. There are some webers / kits which claim (no first hand knowledge) to be emissions compliant for select year models of toyota. Don't know about isuzu. They (weber) pretty much state they're not emissions compliant. Minimally, you'll probably need the fuel shut off solenoid. If they don't lift your hood and only test "emissions", there's ways to legally past the tests. The ecm in your 90 model is very primitive therefore it doesn't take much to meet or exceed its emissions control/output. Do a little searching and you'll find what you need to do fuel wise, engine temperature, etc. One other thing to take note of is the weber is intentionally jetted rich. This alone will impact the emissions. About the only difference in your california emissions / federal emissions control is the california emissions vehicle has an exhaust temperature sensor feeding data to the ecm whereas the federal versions do not. There might also be a small difference in operating temperature idle speed (rpm).
1989 Isuzu Trooper S/ 4WD / 2.6 Litre 4 Cyl 4ZE1 Converted to Weber 32/36 Carb / 5-speed Manual (MUA5) / Warn Premium Hubs / Fancy Radio / Deluxe Intermittent Combination Wiper Switch
I'd rather laugh with the sinners than cry with the saints.