It will probably take a lot of time to mill this and put quite a bit of stress on the lathe. That is a lot of material to remove. I would suggest you do some layout and either bandsaw or hacksaw away as much steel as possible, leaving a small amount to mill away with better precision. Due to the amount of curved surfaces, a lot of filing will be required.
Alternately, you may be able to cut/bolt/weld together a bunch of rough pieces of metal to make a temporary replacement and use the mill to make the actual replacement. I would probably take this route.
