Why were you sleeping for so many years and why did not pressed for the document to be rgistered.
If you have paid the full consederation amount and the receipts are duly acknowledged by the seller ,then you can file for declaratory suit and you can make the LR's of the dead seller as parties. However be careful the LR's are going to get consious and may creat problem or would feel greedy to make u pay more if they are not having moral charecter very much live. You are in a strange situation,
You have not mentioned wheather the land is agricultral or other. You should carry on peacefull posseion.