Radar measured things much slower then + Rackets speed things up
In the 1990's forinstance, Boris Beckers top serve was listed as 131. Now everyone and their dog is serving this fast. Are they faster than Becker? I don't think so.
About the Mid to late 90's things seemed to change. Until about 1990 the top serve was around 135mph. And now quite a number of guys are serving faster than 135, and a number in the 1940's.
McEnroe - when he played doubles in 2006, was clocked at 119. Now you have to believe that a 1984 McEnroe is going to serve at least 10mph faster than when he was nearly 50.
Hence, in answer to the question, how fast was McEnroe's serve. It depends on what radar clock and with what racket you use. Using today's clock and rackets he would be approaching 130 for his fastest serve.
Surely its not hard to conclude that if can serve around 120 now, then he was capable of 130 using today's equipment and radar guns.