Well, we never do get any details of the process of installing the engine (as I recall all Karlach says about how she survived it is "high pain tolerance"). My point is just that none of Karlach's story could happen in the first place if it wasn't possible to replace a heart with some kind of replacement without killing the person involved. I wouldn't expect the process to be simple, of course- you'd need either very powerful magic, help from a powerful devil, or both, in all likelihood- but then, neither of those is exactly a huge stretch within the game. Maybe it is actually impossible without the power of an archdevil, but there's nothing to specifically suggest that that's the case. It could be- hey, that might even be an explanation you could give the player for why it can't be fixed, if we'd actually been given one- but based on what we are told by the game right now, there's no reason to assume it *has* to be impossible.
(There's also the half-joking point to make that it might not even matter if the process does kill her- Withers and/or raise dead scrolls work fine on Karlach, so as long as you could get the engine installed in her corpse, you can just bring her back to life after. Unpleasant for Karlach and more than a little silly, definatly not anything I'd like to see them implement, but the game clearly does establish that raising the dead is possible, so...)
EDIT- just noticed the post above me, wanted to say I love the idea of bringing a fixed engine to Avernus with the intent of forcing a devil to install it as a way to make the Avernus ending much less bleak.
