The unregistered sale agreement is not valid in the eyes of law.
You say that you do not have physical possession of property neither the revenue records are there in your grandfather's name.
Perhaps your grandfather would not have pursued this matter or he might have not shown any interest in purchasing this property during his lifetime for the reasons known to him or he may even had lent some money to the seller for which this sale agreement was executed.
In any case the case is very badly barred by limitation.
Moreover the land is occupied by someone, perhaps he might have purchased this land by a registered sale deed, hence it would be better that you verify the EC also in this regard.
In any case you have very less chance to win even if you propose to take it legally.