I mean a modern Rider with a modern (but legal ) set up pre 75 bike .
For Example Harrie Everts son ( also a world champion) riding his dads bike that we have modified .
30 years worth of human evolution is irrelevant.
30 years worth of riding development is significant.
30 years worth of bike development is huge, even if we apply that development to a VMX-legal bike.
I have no doubt that if you build Stefan Everts a good, 2008-spec VMX bike, gave him time to learn to ride it, and transported both the bike and him back to 1974, that he'd wipe the floor with the heroes of the day.
Which is not to say that the heroes of the day weren't exceptional riders. While we're on the hypotheticals, I reckon that:
If you took the top riders from 1974 and put them into a time machine and brought them to 2008, many of them would rise to the top among the current crop of riders. The will to win, and the natural talent is what makes a top rider, far more than anything else.