His athletic ability is obviously his strength, but had he been born to play in previous eras, he would not have the same physique as he does now. I mean what Borg put in at the gym was the gold standard at that time, and how things have changed since then. At the same time, he has got to be one of the most heavily benefited players in terms of current technology. That topspin forehand isn't going to affect someone with a similar backhand weakness such as that of Federer in previous generations especially with wooden rackets.
it's arguable either way about how good shape he would be in another era. he is the equivalent of BOrg as far as fitness compared to his fellow generation, so I think he would still have a massive advantage physically no matter what time he played. I do agree with the racquet argument, wooden rackets would be difficult for him to consistently win with and he'd be even easier to attack than he is now. but latter 80s, 90s, I think it's irrelevant the differences, he would still dominate on clay with graphite racquets especially when the large frames were introduced in the latter 80s