Your character could survive indefinitely in an intensive care unit after having all the body parts he doesn't actually need replaced by gold prothesis, to the point where half his volume is gold.
He should be full of antibiotics and in a clean room to prevent infections.
Would best fit the mood of a horror story.
Replacement would definitely need to be done as surgeries by very good surgeon teams in a hospital, over weeks or months. And, again, intensive care forever. All involved would need a good reason to participate, and most jurisdictions would need a good reason to allow it.
Parts to replace:
-Arms and legs, as far up as the doctors can manage.
-Bones. Ribs should go. Maybe pieces of the skull. Definitely all teeth. Remember to leave some bone marrow to produce blood.
-Ears and hair would definitely be gold.
-Eyeballs. Who needs eyeballs?
-All sorts of cartilage.
-Genitals and buttocks. Rerouting pee and poo through holes in the torso will probably be more practical than preserving natural openings for them.
-You cannot replace all intestines, but you can preserve a working percentage of them, ending in that hole in the torso, and replace the rest.
-One gold kidney. In a pinch to meet that 50% goal, you could replace both and also depend on dialysis.
-The oesophagus and tongue. They would no longer be needed as a tube could pump pre-digested food directly into the stomach.
-One gold lung. You could survive with only one, an O2 tank could be used to supply additional oxygen if needed.
-Nose. Why would you need a nose? All you’d smell is gold anyway.
-The Appendix. Simply a storage facility for digestive bacteria. Besides, they often only cause problems.
-The Gall Bladder. You can live without one of those too.