You need to get the sale deed registered without which you don't have clear and marketable title to the property though you have other documents to prove your possession and enjoyment.

You first issue a legal notice to the seller to his last known address and get the registered post notice returned undelivered.

After that you can file a suit for declaration of title and for execution of registered sale deed through court on the basis of documentary evidences held in your support.

It’s mandatory to do the same you can also register it belated by paying penalty 

Deed of confirmation has to be executed by seller and duly registered 

if seller is not available then it cannot be registered 

you can file declaratory suit that you are absolute owner of flat 

Dear Client

1. Registration Not Possible Without Seller: As the contract was not registered and the original seller cannot be traced, you cannot register the older contract at present or get Index II since registration is only possible with the presence or legal representation of the seller.
2. File for Adjudication & Seek Regularization: You may file an application before the Sub-Registrar or Collector of Stamps for adjudication of your case under the Maharashtra Stamp Act, requesting permission for late registration or a declaratory order on the grounds of continuous possession, payment of stamp duty, and recognition by society.
3. Alternative Legal Route – Declaratory Suit: If adjudication is not feasible, go to a civil court and file a declaratory suit under Section 34 of the Specific Relief Act to declare your ownership; court's decree can facilitate mutation and other legal formalities including Index II.

Dear Sir/Madam,

When the person is not available and also not possible to register the said agreement as of now. In these circumstances, you may go for a civil suit of declaration the same making the society as a defendant.