I think some very mild regulations took effect in 1969 but they dealt more with gross examples like crankcase vents open to the breeze and vented gas tanks. So 1970 and on Volvos have PCV crankcase ventilation and charcoal cannisters for gas tank vapor recovery. Volvo's didn't need cats and air pumps however, because they used D-jet fuel injection by 71 or 72 (not sure when carbs were phased out on 140s). The D-jet computer had a special idle circuit with a fuel mixture adjustment knob on the side of the computer. At the factory they'd start the car, hook up a exhaust gas analyzer, and hand tune the idle mixture. This allowed it to pass the government text (i.e. at idle ) without extra equipment. However, tightening standards made them reduce the compression ratio (though I'm not sure how that affects emmisions) in 72 with the B20F engine. That dropped the HP from 130 down to 112, but still didn't require any additional equipment besides the hand tuning.

And while 130 sounds pretty good, don't forget that more modern engines do much more with much less displacement. Then engine in the Honda S2000 is also a 2.0 liter - it produces 240 hp @ 8300 rpm and 153 lb/ft @ 7500 rpm, well over 100 hp/liter without forced induction. The best B20E figures (factory) were 130 HP @ 6000 rpm and 133 ft/lb @ 3500 rpm. The Honda has 8 extra valves and a neat two stage camshaft system (VTEC) with a low end torque cam profile that switches over to a high rpm screamer cam profile at around 4500 rpm.