Those guys were physicaly tough back then. They didn't have all the technology, bells and whistles back then. Those dirtbikes were like the equivalent of an old Triumph streetbike with off road tieres mounted on them.
no lightweight plastic and carbon gizmos and such. Just lots of metal, poor ergonomics and backbreaker suspension....no 14 inches of travel there...
Then there's saftey gear, or lack thereof. those guy's probably ate roost and rocks for breakfast and loved it.
I can just imagine what a racer from that era could have been like on a modern machine from today.