1)Stamp duty is not paid on time, it attracts the penalty at the rate of 2% per month on the deficit amount of Stamp duty. However maximum penalty can be only 2 times of the deficit amount of the stamp duty

2) registration of sale deed has to be done within maximum period of 8 months from date of execution

3) registration cannot be done after period of 26 years

4) your mother should contact seller and deed of confirmation can be executed by seller and sale deed annexed to the deed of confirmation

The stamp duty and registration charges are local subject i.e., as per the local laws.

You can enquire about it locally.

Without a registered sale deed on her name she cannot claim title or ownership to the property legally, hence it is advisable that she gets a registered title deed in her favor by observing the formal procedures in this regard.