* Some crate engines are emissions legal as "service replacements," if installed into a chassis that originally had a variant of that engine and if all emissions control systems are retained. In this case you're using the emissions controlled accessories from the existing vehicle.
* A few crate engine packages have CARB Executive Orders. Due to a weird enforcement note from the EPA, CARB EOs are essentially a loophole in the emissions certification process. These Executive Orders apply only to crate engines that are retained in the shipped configuration and only apply to packages that come with accessories including intakes and upstream-catalyst exhaust systems (usually header+downpipe).
* Exhaust downstream of catalyst is a murky area. It's "de facto" considered not part of the emissions control system, even though backpressure could affect actual measured emissions.
Custom non-road engines are found in many track only cars. And if you are to read the article I posted early, even the EPA doesn't really care if a street car is converted to a track car so long as it's never used on the street. It might be technically illegal to do, but the enforcing agency doesn't care. Pretty similar to things like federal enforcement of marijuana. Not legal, but not generally not enforced.
