It's California. There's no way to beat it.
It needs to have all the original smog equipment hooked up and working.
If it's an engine swap, it needs to have all that model year's smog equipment hooked up and working, and can't be replaced with an earlier model engine.
No aftermarket carbs, intakes, exhaust, etc, unless approved for California use.
It's the visual test that will get you. My first Civic, with dual mikuni side drafts, header, etc, easily passed the "sniffer" test, but had no smog equipment (vacuum canister, egr valve, etc...)
The car has all the smog stuff in it, just not hooked up, so my plan is, after I replace the engine with one of the crate motors, get the stock carb, manifold, etc. Hook everything up legal, and build the other motor up as a race engine.
At the moment, I don't really have the desire to build another race car, just a cool, street-able Civic, but if I wanted to swap out the engines, it'd take a half a day.
It's also not going to be a daily driver, because I have a super reliable 2002 Civic with AC, automatic, power windows, etc.
But it'd be easy enough to make a sleeper Civic, and then swap the stock motor back in every two years for the smog.
Or, find a 75 chassis and build a full racecar that doesn't need to be smogged, and get back into autocrossing.
But I don't really have room for three cars and ten mopeds at the moment.
It was pretty much a nostalgic purchase, and I'm probably gonna end up dumping way too much of my unemployment and stimulus checks into.
I'm already in about $2500 deep. These cars, stock and in good condition can fetch 5k to 15k (for perfect examples) so I'll probably end up with a car that is hopefully worth what I put into it. Minus time of course, but y'know, labor of love and all that crap...