It's interesting that you posted this, as I saw a little of Lendl vs Born in the French Open final yesterday.
With the equipment that they were using at that time, it was a completely different game. It's more about keeping the ball in court and getting it over the net than hitting insane winners, at least on clay.
I prefer the game the way that it is now, but many of the techniques that have developed wouldn't have worked at all. It's hard to say who would be better or worse, as many players would have to completely alter the way that they hit the ball.
It's pretty obvious that Federer would be one of the best, as he's the best all-round player in terms of having different facets to his game, and is the closest to an old-school player. His backhand, serve, and volleying wouldn't need to change very much, he would have to work on his forehand, though.
Modern equipment has largely killed one-handed backhands, serve-volleying, chip-and-charging, sliced forehands, baseliner vs volleyer, different tactics on different surfaces, and probably several other things. It's only really Federer keeping many of those things alive, and when he's gone some of them will be gone for good.
While I still, on balance, prefer the way the game is played now, it has also gone too far in the direction of endless baseliner clones, who play essentially the same game on every surface, and have little craft, subtlety, invention or variety to what they do.
It would be nice if there could be a halfway house, but it seems that the authorities are happy with the way things are currently.