Imagine Borg's power and spin with a modern frame folks, if he could generate the shots he did with his old frames. I submit that it would be quite scary to face him if he was playing with, for example, a modern Babolat when he was anywhere between 20-25.
Think about his serve, forehand, and backhand with that frame in his hands or say a modern Head/Wilson/Technifibre. He could CRANK his serves and his topspin forehand would PUMMEL other baseliners and volleyers alike. Also, how do you think his returns/passing shots would be? The guy was STRONG, so his power/pace/spin would be off the charts. Add on his retrieving skills (he was one of the fastest/quickest players EVER), and play on Clay and all other surfaces and he would be a force of nature most likely.
John McEnroe would also be VERY SCARY in his prime with the frames of today.