[Noah930]

No longer a "rolling exemption" for smog checks. 1975 is the cutoff. So anything after '75 has to get smog checked every other year. Technically, a car only has to pass the smog requirement levels for the date in which it was sold. But there seems to be some debate on whether that's true, or if the DMV/CARB seems to be making regulations more strict with time, and expecting older cars to pass newer (more clean) smog regulations. FWIW, the 3 or 4 times I've had my car smogged, I've had to have met the same minimum/maximum levels each time. But others on this board claim that they've had to pass ever-stricter limits with their cars.

Some will take an early chassis (pre-75) and put in a newer motor, like you suggested.

Smog checks are done by independent (but licensed) shops--not by any governmental agency. But the computer/equipment these shops use is hooked up to the DMV, so any test they run gets logged by some central computer system.

Our smog check not only has a sniffer test, but also is unique in having a visual requirement. The engine bay must appear stock. It must retain all smog and exhaust equipment as it had the day it was sold. Even if your car passes the sniffer test, it may fail if you've disconnected the smog/air-injection pump, or modified the exhaust. Of course, whether the smog test technician detects those changes depends on the acuity of the tech doing the test.

If you buy a California car, it is the seller's responsibility to get the car to pass smog before the sale. (Something like within 30 days of the sale, IIRC.)