Regeneration is a 7th level cleric spell, which is not in the game. For losing an eye, you need regeneration. Or to die and have Resurrection which regrows lost parts, another 7th level cleric spell.
Tbh, I expected to eventually find a high level cleric in Baldur's Gate, but no such luck.
The hag's eye you get does seem cursed to be bound to you. Not surprised 3rd level Remove curse doesn't work, but 5th level Dispel Evil should work, or even upcast Remove curse, maybe with a contested check. Guess it doesn't because reason.
