Replacing the centerboard with lead was a terrible idea that has probably pooched the boat's stability numbers. Cutting out the centerboard trunk and massively glassing in a keel stub, and then through-bolting a lead wing or fin keel would be a better idea...if a naval architect was involved. The scantlings of the hull might not support the forces at that point.
This idea seems so dodgy I would avoid this boat like the plague. Either have the original centerboard replaced (maybe with a design refinement that allows it to "kick up" if smacked) or just walk away.
Would you buy a classic sedan that had wooden wheels with iron rims on it because the owner "kept getting flats"?