A few of today's athletes might have been competitive 50 years ago, but most would DNS.
First, there was no financial incentive to race. Even the top runners needed to keep a job. That would eliminate all the East Africans.
Second, energy gels didn't exist. Posters who complain about PEDs don't realize that GU revolutionized the marathon. In the 1990s, runners used to joke about their pre-Gu times vs their post-Gu times. Without Gu or another energy product, some of today's runners wouldn't be able to hold pace for 26.2 miles. In the old days, only runners who COULD race without energy gels were at the top level.
Third, training methods, poor shoes, and primitive medical interventions would have resulted in injuries that would limit or even end running careers.
Fourth, runners who are pushing the limits of legal performance enhancement, which includes a lot of the world's best today, might drop back to sub-elite if they were forced to give up their thyroid meds, asthma inhalers, B12 shots, altitude training... and on and on.
Fifth, and then we have the ones who are on PEDs today. They obviously wouldn't be competitive.