File Police complaint about loss of original documents 

2) issue public notice about loss of original documents 

3) apply for and obtain certified copy of said documents from sub registrar office 

If un registered documents are missing then approach original seller and obtain copy of the said agreement 

See after sale deed is executed sale agreement is of no use , further though for both the agreements file a police complaint, take a lost certificate from the police, public advertisement in newspaper it will be enough for future sale.

Sir follow the procedure as mentioned above in case they are lost or missing.

File police complaint, there wont be any issue due to loss of these documents. 

Approach the builder to certify xerox copy of said agreement 

You need to file police complaint in the same. Lost Unregistered document can't be retrieved as there is no record of the same with sub registrar. You need to give a paper publication in newspapers for the lost deed and approach court if necessary for the same. If the earlier parties of deed are alive it can be again  executed by paying necessary stamp duty and registration

Unregistered document has no validity and neither it will effect chain of documents. Agreement of sale constitutes the terms and conditions of sale of a property by the seller to the buyer and once sale deed is executed, ATS expires.

Keep xerox for record.

if agreement for sale is not registered then you can enter into fresh agreement in which you can mention about the loss of documents, FIR, Advertisement and simultaneously you need to take indemnity cum undertaking bond from the seller.. 

Since you have the xerox of the agreement there is nothing better that you can hope for in view of the fact that sale agreement is unregistered. 

If you check all mutation papers you will get from revenue department copy of it.

You should apply for true copy of registered agreement from registrar office. 

But before that lodge FIR for loss of original documents.

The unregistered sale and construction agreement is also a vital document.

You may ask the seller to furnish the photo copy of the same and also an indemnity bond indemnifying against the losses if any due to this in future.

You are repeating the same question in a different manner, but the reply is the same, in my opinion, it would be better that you obtain an indemnity bond for future security.

You can obtain an indemnity bond indemnifying against  the losses in future from your vendor

You don’t have clear and marketable title to property as original chain of documents are missing 

Unregistered sale agreement is of no use.

You can always seek relief from your vendor in this aspect.

You should not be concerned about the builder or any predecessor to your vendor.

You have to look for remedies that is available before you and not go beyond that, there may be many reasons that may be an obstacle to your proposed move.

You must concentrate on your relief through your vendor, let him say that he cannot do i, you can take legal action against him as per law.

You should consult a local property lawyer. 

see there won't be any issue the agreement is not required.