The remedy is to execute a rectification deed, but this is possible only if the seller agrees to do so. So if the seller is deceased then his legal heirs can execute the rectification deed in favour of the buyer or buyer's heirs. You should trace and contact the heirs of the seller to convince them to do this. If they refuse to cure the defect then a lawsuit can be filed against them in the court to seek judicial directions to them to execute the rectification deed.