>To me it seems unnecessary to have a donor body: might it not be
>possible to replace it with a heart-lung machine?
Not yet; you'd need a liver and bone marrow as well. Further, although
the intestines and the spleen could be replaced, I don't think the
replacements are yet of the caliber for indefinite lifespan.
Also, current heart-lung machines require a threefold dilution of blood
to operate and that would leave your hemoglobin too low to function
properly. Finally, the current versions chew up red blood cells at a
rate far exceeding the ability of the body to replace them.
I think cultured organs would be much more practical than body transplants.
I think we'll be able to grow hearts and hook them
to the appropriate nerves long before we can build an interface
between two different spinal cords.