Property is not registered by the seller with the registrar

Hi, The Flat that I ('prospect buyer' of the property in query) am looking to buy was not registered by the current owner ('prospect seller') with the sub-registrar. Nor was the stamp duty paid by the current owner then. Property was bought by the current owner in 1986-87. I have not yet entered into an agreement to buy this property. The original copy of the property agreement is lost and a xerox copy of this is available after registering FIR with local Police station. Notice has also been given in newspaper as per procedure. Now that I am applying for Home loan the bank is seeking a Certified copy of the property agreement which is not available due to non-registration. The owner of the property is deceased. The ownership right of this property is with the Legal heir (son and 2 daughters). I wish to understand what is the remedy I have if I want to buy this flat. What actions are required from the current owner (prospect seller) as well as on part of the prospect buyer (me)? Thanks in advance