I'm sure we all understand that the emissions regulations at issue in Dieselgate don't apply to these vehicles, as they don't apply to off-road, racing, etc. Emissions standards, for better or worse, are not retroactive.
+1
It is my understanding that by federal law the car should comply with the emission standards at the time the car was built, and (if you're very picky) to the configuration it had after leaving factory (so you can be spewing unburned gas and be ok, as that was the standard at the time...). That is good for avoiding retroactive liability to car companies. Still, changing the engine or the carb, camshaft, compression, etc... can bring you trouble if the car is registered as a daily driver (in theory, and probably in California, the car will require a pollution inspection, and won't pass unless it produces the same or less contamination).
However, (depending on the state I think), if it is registered as a Classic, Antique, Off Road, Etc... you don't even have to comply with that, but there might be some restrictions on the car use (i.e.: in TX a car registered as an "Antique" should be used only to drive to and from car events/shows, or for maintenance requirements). That will keep the contamination levels low enough.
Enforcement of these laws might be lax due to the love for cars in this country, but I wont count on it, so I wont drive/recommend driving an old modified car as a daily driver if it did not pass the emissions inspection, specially in California.
I will consider this as thread drifting, so I'll refrain to write more about this...