Wondering about the 49cc vs 68cc

I love how cars built before the late 60's dont even need to be smogged. You can have the nastiest rat rod with 4 barrel carbs and a straight pipe that gets 7 mpg at 80 decibels no problem... but the second you want to put a 66cc engine instead of a 49cc on your bike it becomes questionable... WTF?????
 
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