Unregistered agreements

I want to buy a flat in Mumbai. The owner doesn't live there and has been giving it on rent for almost 29 years. He wants to sell the flat now. The building was constructed in 1984. Here are the details: 1. Flat was allotted to an army serviceman Mr. X. Allotment letter original available. 2. He signed an "Agreement For Sale" with builder in 1985. This agreement is neither registered nor is stamp duty paid. Original of the agreement is not available. I got a copy of the old agreement from the builder's kin. He was kind enough to give me a copy but did not agree to give me the original version that he possesses. 3. Society's Conveyance Deed (Deed of Sale) signed in 1987 has names of all flats and members. Mr. X's name is present against the flat. 4. Mr. X sold the flat to Mr. Y in Dec 1991. An "Agreement For Sale" was signed by both parties. Stamp duty was paid. But it is not registered. 5. In 2021, I'm trying to buy this flat from Mr. Y. He has been giving this flat for rent all these years. He has been treasurer of the society in the 90s and has been paying society maintenance all these years. There are no dues. 6. Mr. Y found out that the share certificate back side doesn't have his name. This was added by the society in 2020. 7. Society has given NOC for sale. I really like this flat and want to buy it. What should I demand from the seller Mr. Y to get the title clear? SBI is asking for registered mortgage for home loan which is costing me a lot of money. Thanks!