You should get a certified copy of sale deed from the office of the Sub Registrar. A Registered document like Deed of sale is not a public document. It is a private document. Book 1 kept in the Registration Offices under the Registration Act, where the Registered documents (private documents) are copied, entered or filed, is a public document.
A certified copy of a registered document, copied from Book 1 and issued by the Registering Officer, is neither a pubic document, nor a certified copy of a private document, but is a certified copy of a public document.
According to section 17 of the Registration Act sale of immovable property valued more than 100 rupees must be registered. That registration is a notice that property is conveyed from one person to another. According to Section 51 of Registration Act, all deeds relating to immovable property of which registration is compulsory under Section 17 are entered or filed in Book 1 kept by the Registering Officers.
Section 62 of the Registration Act provides that Registrar shall, on receiving requisite fee, provide a certified copy of the entries made in book 1. Such certificate (certified copy) shall be signed, sealed and dated by the registering officer, and shall then be admissible for the purpose of proving that the document has been duly registered in the manner provided by this Act.
Thus nature and evidentiary value of the document remains the same whether it is a registered deed or certified copy of that deed. Section 77 of the Evidence Act provides that the Court shall presume to be genuine every document purporting to be a certificate, certified copy or another document, which is by law declared to be admissible as evidence of any particular fact and which purports to be duly certified by any officer of the Central Government or of a State Government.
Above mentioned that a certified copy of a registered sale deed issued by the Registering Officer, by copying from Book 1, is a certified copy of a public document.
A certified copy obtained from a Registrar’s office shall be admissible for the purpose of proving the contents of the original document. That means that the mere production of a certified copy without any further oral evidence to support it would be enough to show what the original document obtained.
A certified copy is called secondary evidence of the original. Section 65 & 66 of the evidence act provides that if the original document is lost, a duly certified copy of the original shall be produced and it is admissible in evidence.
No problem will arise in future that the original sale deed has lost and the owner possesses only certified copy. FIR is not a substantive piece of evidence to prove that original sale deed has lost, it is a conduct of the informant that a relevant point of time he admitted that original sale deed has lost.
You can file a declaratory suit under section 34 of the Specific Relief Act and get a declaration from the court that the original sale deed has lost and you are the absolute owner of the property. This suit shall be filed by you against the previous owner (seller) of the property. He will accept in the court that the original sale deed has lost, he sold this property on lawful consideration and transferred absolute right to the purchaser (you).
That declaration decree shall be conclusive proof of the facts declared by the court. However, it is not necessary to file declaratory suit but you can file it for the better security of your title.