Issue public notice about loss of original documents 

you have already applied and obtained certified copy of documents 

in addition FIR has been filed about loss of original documents 

The first step you should go for, after filing an FIR, is to collect the non-traceable certificate from police, which shall state, that despite various attempt and investigation they cannot trace your lost deed.

Then Publish the advertisement in a lost and found column in a daily common newspaper, if possible in both English and Hindi newspaper, stating the particulars and all relevant details of the property. 

Then write an application to the office of sub-registrar, as per the jurisdiction, for duplicate deed. states all the facts in your application and don't forget to annex the Non-traceable certificate issued by police and the copy of the publication in the newspaper.

Dear Sir/Madam,

You are suggested the certified copy obtained from the authorities are as good as the original one and you are required to do nothing further. 

You need to seek a declaration from the court for the same

duplicate copies into pakka documents - Not possible. 

1. Apply for certified copy of conveyance deed and lease deed and/or under RTI. 

Certified copy, missing report and newspaper these are only formality required to sell property. 

Get an FIR lodged.  Merely  filing of  complaint  with police is not enough.  Secondly, apply for certified copy of CD from the office of sub registrar concerned from where the CD was registered.   File a suit  for Declaration to the effect that you (grandfather) are the owner of the property  and the CD has genuinely lost and certified  copy be read as original to  deal with the  property  in whatever manner  your grandfather wishes. It is a settled law.

Besides complaint with the police and public notice about the loss, you should obtain a certified copy of the registration document from the sub-registrar office and keep it along with the copies of the other relevant documents so that this will become an authentic document.

Your grandfather can execute a Gift deed in favour of his son or children or any family member jointly or in favour of anyone to make registered original deed of property.

Upon releasing the notification in the news papers i.e., one in vernacular and the other one in English Language informing the general public about the loss of original documents and not to deal with the property in case any one finds them.

Further, based on the above, publication a complaint has to be lodged with concerned police, based on which they will furnish report, using the same, the concerned authority will issue duplicate title deeds.

Don't get the duplicate copies. Get certified copies 

file police complaint regarding missing of documents within whose jurisdiction documents missed. Give paper publication, Notarised the same fact.. Apply to Register for certified copies. 

there is no way by which the duplicate copies can be converted into pakka documents

duplicate can at the best be certified as a true copy because the records of the documents would be available with the sub-registrar with whom the documents must have been registered

however you can explore this option as below

your GF can file a civil suit against unknown persons claiming a decree for a declaration from the Court that he is the absolute owner of the property basis the certified true copies obtained from the sub registrar office

the court will then require your GF to issue a public notice informing the public that such a suit is filed and the court will pass an ex parte decree in the suit in favour of the GF if within the notice period as specified therein no objection is received

as the suit will be filed against unknown persons, it is obvious that there will be no defendant and the matter will proceed ex-parte 

once the notice in the above public ad expires, the GF can then apply to the Court to pass the decree

the court will thus pass the decree

in the suit it can also be prayed that the decree be registered in the favour of the plaintiff by the sub-registrar

this will serve as a title document in the hands of your GF

in the alternative the court can appoint some suitable court officer who will execute and register the title deed in favour of your GF on strength of the decree passed by the court

the above procedure will atleast ensure that your GF has some original title document in his hand

it can however never replace the originals which have been lost 

but something is better than nothing

and if there is a court decree then it becomes easier to sell when the buyer is funded by a banker 

- If you got the certified copy from the authority , then on that ground you can sell the property to anyone, and only a duplicate copy is not enough. 

- You can apply for getting certified copy of the said lost documents before the authority .