AJ also says all greats would find a way in any era, I dont fully agree with that statement (Rosewall and Mcenroe would not find a way in the modern era, even Sampras doubtfully would achieve much, that is what I think atleast). However, I see absolutely no reason to question Borg's ability to learn a great forehand with 90s raquets or modern raquets, ofcourse it would be great and he would love to play in the modern grinding era of the 2000s, even if Djokovic and Nadal probably would give him a hell of a fight.
Anyway, Borg would beat Courier in any era on clay, that much I can say without any doubt.
Players can only be judged by their achievements in the era they played in. It is all that simple. True some eras are stronger than others, but one can only play those who are available at that time. Rosewall played in very tough eras as an amateur, pro and the open era. He was a top 20 player for at least 20 years. At 5-7, he acknkowledges that he would have a very tough time in todays game. Still, he as an all-time great for what he achieved in eras he competed. Sampras at 6-2, would have been very successful in today's game using today's equipment. The same is true of Agassi, Courier, Edberg, Becker, Connors, Borg, Lendl, Wilander and a few others.