... if they had to play against current players, presumably using current equipment.

I ask the opposite question: What if today's players had to play against the Laver-Emerson-Rosewall-Connors-McEnroe group using the equipment from the older era?

The equipment differences lead to differences in grip, swing, spin, timing, etc. Today's players would have to learn a whole different style to play with old equipment, and yesterday's players would have to learn a whole different style to play with today's equipment. It takes years in either case. You could not just practice for a month and then play someone who has been playing with certain equipment for 15-20 years.