Try explaining that catalytic converters were not used until 1975. The cop might have assumed that the loud exhaust meant that the cats were gone. Take a copy of the Owners Manual and or the FSM pages that show that there is no catalytic converter. Don't talk about anything under the hood. Not a word.
Also point out that the car was properly tagged and registered with the equipment the cop cited as deficient. And apologize for your brother's stupidity.
https://sciencehistory.org/stories/magazine/clean-machine/#:~:text=Beginning with the 1975 model,%2C nitrogen%2C and carbon dioxide.
Three years later the recently created Environmental Protection Agency mandated that all cars beginning with model year 1975 be outfitted with catalytic converters to meet emission standards. Thousands of researchers were put to work by catalyst companies and car manufacturers. General Motors alone, at the time the country’s largest carmaker, put 5,000 people to work on catalytic converters.