You could "regenerate" a person's "missing organ" through a "process" called "transplant".
Erm, no, you can't. That's not regenerating their organ, that's sticking a completely different organ in there which happens to work just as well.
In a similar manner, you could theoretically perform a brain transplant (certainly with infinite time to design the equipment, anyway). It would just be somewhat pointless and
extremely unethical (as well as most likely not working for any length of time, because if your immune system starts attacking your brain you're fucked...).