If it's the alloy Altitude 50 you want an
XT Direct mount FD-M771D.
Any 30.9mm seatpost will work (and an adjustable seatpost like a Crank Brothers Joplin can be useful), a 34mm seatpost clamp would be needed, and your current headset could be reused on the aluminium Altitude frame.
If it's the carbon Altitude 50RSL, the same front derailleur and seatposts would be needed, but you don't need a seatpost clamp it's built in. I am a bit puzzled, on the current photo of the Altitude 50RSL on the website there's a conventional headset in the photo, not the Campy compatible integrated headset used on the Altitude 70RSL and Altitude 90RSL.