If he used what I’ll call (for the sake of argument) paranormal abilities, things that surpass all known avenues of causation, it’s still theoretically possible to manipulate your own genes to become immune to major diseases, and also to alter (suspected) ageing factors like shortening of telomeres, genetic mis-copying, etc.
I mean, on top of the totally paranormal stuff that I also completely think is possible, and even think I may have done on occasion.
Or else, someone who achieved a state of just not dying would die around 600-700 years old, because statistically, that is the maximum statistical chance that you die from slipping in the shower or getting ran over by a drunk driver.
Then you’d need to alter your own luck, “hamingja” - interesting Wikipedia page btw:
"In Norse mythology, hamingja (Old Norse “luck”) refers to two concepts:
the personification of the good fortune or luck of an individual or family, the altered appearance of shape-shifters."
If to accept a donor heart, is to take on some small part of the personality of the donor, which seems to happen quite often, and therefore (if you believe our thoughts and emotions often dictate what happens to us) their luck; if to shapeshift into that person (either through bodily re-writing or forced possession/reincarnation) is like an extended version of that, then if I became you (since you’re slightly younger than me I think) - I’d just be you, if not right away, then eventually.
All of which is a bit random, but does argue for incarnation within eother a set of genes that promote the personality you want, or prolonged life in an existing body that already creates/manifests the personality you want.
Apologies if I’m rambling a bit.