That's a lovely bike as is...... I echo what Ameybrook has said, go for a nice XT or XTR rear mech. Perhaps an older (but fully refurb'd) Manitou fork, maybe a Flite saddle, and it's more or less good to go.
I really want something closer to period correct. maybe a p2 or something. I looked into getting fork blades made for the mag 21. They look pretty rad.