How to compute fair value of property as on 01 apr 1981

"Is valuer report required for the same?"

Yes, approved by government.

The date has changed from 1.4.1981 to 01.04.2001; vide finance bill 2017.

Thanks  Sir, from where i can check the valuer which is approved by the Govt.

Many professionals like CA, Architects, Engineers are approved by Goverment for such job in different areas, whose valuation report is accepted by Governmental departments. May check with Sub-registrar office of your jurisdiction.