Registration of Agrement for Sale & Exchange deed after expiry of owner

I entered into notarized MOU (Memorandum of Understanding) for purchase of one flat and exchange of second flat with a Seller having two adjoining flats in CHS building in Mumbai. Mine flat which is to be exchanged with the seller’s 2nd flat is also in the same building. Seller had mortgaged his both flat and taken home loan and also had pending dues of society maintenance charges against both the flats. As per mutual agreement with the seller, I have cleared all his outstanding dues of home loan and society maintenance charges. It was mutually agreed and also as per the MOU terms and condition after completing aforesaid payments seller was suppose to execute and register with respect to the 3 properties. (a) Registration of Agreement for Sale (b) Registration of Deed of Exchange. Unfortunately, seller expired before completion of deal as per MOU terms. Property -1 which is for exchange seller was the sole owner and for property -2 which is for sale joint owners are there First owner the seller and second owner his wife(who is alive). Kindly advise me how to proceed further for registration of both flats.