There's no tax on inheritance. Hence, no need for valuation.
When you will be selling this property, cost of acquisition for your mother shall be considered as your cost of acquisition.
Since property was purchased before 1.4.01 , you need to get a valuation done as on 1.4.01 which shall be deemed to be cost of acquisition and indexation of this cost shall be done upto the year of sale.
Indexation shall be done with Cost Inflation Index.
Example, property purchased for 1 lac in 2001-02 shall have an indexed cost of acquisition of 2.89 lac of sold in 2019-20.
Sales consideration less Indexed cost of acquisition shall be the capital gains.
We may assist you with return filing, valuation and other compliances.
Further, if your mother had income during April-dec 19, you need to file her ITR as legal heir and then surrender her PAN.