Dear Mr Asish Raheja,
First of all i would appreciate your uncle and aunty for their noble act of adopting a child.
Here is what i would suggest, Given that whereabouts of the parents not known, you can access any adoption agency (certified by govt of india,CARA) and explain the scenario and they can help you get this adoption done legally.
Especially in your case agency can take a stand following the procedure for "adoption of an abondned child" thereby the birth certificate will be taken care by the agency and CARA.
Please find the procedure below.
https://adoptionindia.nic.in/guideline-family/Pre%20adoption%20procedure.html
LIST OF AGENCIES:
https://adoptionindia.nic.in/cara-network/SAA.html
HOPE IT HELPS.
Thanks,
Chellapandian.k