Complex sentence: The 1970s saw the decline in muscle cars due to higher insurance premium and tighter emission rules, Yenko began to modify Chevrolet Vega with spoiler, turbocharger and design graphics.
Split into simpler sentences: The 1970s saw the decline in muscle cars due to higher insurance premium and tighter emission rules. A bit later in the decadeYenko began to modify Chevrolet Vega with spoiler, turbocharger and design graphics.