Actually I found something : on the ITF website, if you look at the "calendar", you cannot get the draws of the tournaments before 1991
... but if you search for a player (for instance here Noah), you can get some results in the "activity" file.
I don't think you can get every result (for instance maybe Noah played more junior tournaments than what's written, and I got nothing for John McEnroe ... despite he won RG junior tournament)
but you can get something :
for instance for Noah I found that he lost in Round 16 of Wimbledon 1977 to Van Winistky ... who eventually won the tournament :
(you have to change the dates : here 01/01/1976 to 01/01/1979 for instance)
As I said, at least a hundred websites say that he won Wimbledon junior that year