I don't know what they're made of but I do know that my Mazda which sits right next to it and is also undercover shows no signs of rusting at all so whatever alloy Morgan is using it is definitely not very corrosion proof
kirkusblog.com 2016 M3W John A Prestwich Special 1952 Ford 8N 1968 Honda CL175,and 2018 Ural Retro