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.
The carbon swingarm ETSX-70 had a 419mm chainstay and the alloy swingarms on the older ETSX's were a 425mm effective chainstay length, and the Altitude has a 428.7mm chainstay so that would only be a 6mm difference in chain length compared to alloy ETSX or a 19mm difference in chain length with the carbon rear ETSX. So that's either a 0.5 chain link or 1.5 chain link difference.
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