I don't care too much about hot-rodders either. California specifically requires the original emissions equipment remain intact. Here's two cases where that fails:
1. Close to 20 years ago I read about someone who converted a car to an EV with an old electric forklift motor, but then couldn't register the car. It was a model year that still required smog checks, but it couldn't pass a smog check because the original emissions equipment wasn't installed anymore.
2. My brother inherited our dad's 1992 pickup, and tries to keep it in running order mostly out of nostalgia. He would like to replace the engine with a newer model that would burn less fuel, produce more power, and correctly installed, no doubt would have lower emissions. But then it wouldn't pass the smog check, because it wouldn't still have the original emissions equipment.
Having said all that, I agree that it disproportionately impacts the poor, because the poor tend to drive older cars that are more likely to need repairs to pass an inspection, and because the inspection fees make a larger impact on the budget of the poor, and because the employment flexibility to be without their car for half a day for the inspection, or longer if repairs are needed, is not as common among the poor. You could subsidize the inspections for low value cars, which would help with the cost aspect, but I don't know a way to solve the other aspects beyond trying to find the minimum amount of inspection that meets the policy goals.