As for the original question, 55yo Borg would most definitely not win a game against Nadal.
As for this racket technology discussion, there is one problem.
We only know 2 things: We know how prime Borg played with a wooden racket and we know how Rafa plays with a graphite stick.
Obviously, they have never played each other. However, some have seen these players many times and we can all watch their videos. Therefore, we can make a somewhat educated guess about how a match of these two would go. I see that this scenario is not fair, but it is one you can have a somewhat serious conversation about.
On the other hand, we'll never know how (young) Borg would play with a graphite stick and how Nadal would have played with a wooden stick. If they had played at a different time, they might have developed an entirely different style of play and different techniques. Every player is to a great extent a product of his environment. How they play(ed) is/was a reaction to the equipment, courts, opponents they faced etc. Both are obveiously very talented, so they could have certainly adapted to different situation. Thus, this all becomes pure speculation and the 'equalizing' argument, saying they would have been x% better/worse with cerain equipment is highly flawed.
What we could possibly find out is how Nadal plays with a wooden stick (as a one time try), in case he agrees to play a exo with wooden rackets