There are many spells availabe in base 5e that could easily fix karlach... Reincarnate would give her an entirely new body, many ressurection spells would replace lost parts.
I recommend implementing these choices, with explanations for why they wouldnt work.
Chief amongst these explanations, is that if she dies in a way that destroys her body... her infernal machinery in her, should turn her into a soul coin. Thus rendering her unable to be ressurected by traditional means.
Thus creating a completely different story "how to bring someone back from being a soul coin" for Karlach.
