My grandfather had purchased a land & built a house on it. But has lost the sale deed papers or regi

My grandfather had purchased a land & built a house on it. But has lost the sale deed papers or registery neither he has any copy of the deed. The same is also missing from the concerned government department. But we have approved copy of the map in the name of my grandmother. Also we are paying house tax & it's receipts are also available. Now I would like to know after writing his will weather the issue of ownership will be fixed in his name and there will be no issues in future related to Chain of Documents of the property as it is self acquired property. Thank you. Regards, Gaurav