We can never know. But the 1990s clay scene had very little serve and volley. The clay scene at that time was actually an early sign of the sort of gruelling matches we see today, full of engrossing rallies. One guy definitely harmed by the more modern game compared to 1990s conditions is Lleyton Hewitt. He loved playing against serve and volleyers who pressured him at the net.
Nadal in the 1990s would have played a lot more on clay and a lot less on hardcourts.