I have a 1987 Mirage 25 myself. To my knowledge there is no manual per se, but I have a bunch of papers that I could probably scan and send to you (you'll have to provide me with your email address).
There is also a Mirage discussion group on yahoogroups.com (group is called "Mirage-Kirby"), and they have a huge ftp site with all kinds of data (ftp seems to be down right now, but that should be temporary). Subscribe to the group, then search the discussions for the ftp address and username/password.
Have fun with your new boat!