Notorised GPA For Signing In Front Of Subregistrar Sales Deed Coowners

I am buying a flat which is ready to move but unregistered from Person X and Builder Y. Recently we got the assignment agreement done. Me, my wife and my mother will be the co-owners of the flat. In the assignment agreement, all 3 of us are mentioned and signed. My mother is in Maharashtra. Now the sales deed will happen at sub-registrar between me and the builder Y with person X as witness. Due to Covid situation, my mother cannot travel to Bangalore where flat is located. The builder gave us a reference notarised GPA copy, based on which I asked my mother to make a notarised GPA and send me. Now I am reading that GPA has to be registered without which it's not valid. In this COVID situation nor her nor I can meet at a common place to get this registered, so the question is will notarized GPA will grant me the power to sign on her behalf in front of Sub-registrar. Will sub-registrar allow this. What is the alternative.