First original agreement which was not registered lost & unavailable

If first original agreement which was not registered(YEAR 1989-90)is not available and if I want to buy the same flat what would be the legal consequences. I would be the third buyer. I brief you as below. 1.First buyer original agreement is missing. Said agreement was not registered because in the year 1989-90 it was not mandatory. Stamp duty was paid.Photo copy of the agreement is available. 2.Said flat was sold to wife and husband (Second Buyer) joint name with wife as first name in 1999. Flat was registered and stamp duty paid.All original documents are available. 3.Wife and husband both have expired. Now said flat has been transferred to his nominated Son. He has one sister. 4.I want to buy this flat. I approached schedule bank for loan. They refused to sanction the loan, as they are asking for original copy of first agreement which is lost and say that nominated Son is not successor. Kindly advice.