Gift , Transfer of Property in Mumbai

My dad is based in abroad ( for last 10 years plus ) and is aged 90 and in poor health. He stays with me in the US. I am working in the US for quite some time now, I now hold US Citizenship and am resident in USA. My dad ( not a US citizen) has a property in Mumbai where in he has willed the property to me in the will. However to avoid the probate related issues after death, he intends to complete the transfer now itself to me. 1) Could you please advise which option would be advisable: a) Gift deed or b) Sale deed 2) Please advise of complications in doing the above since I am now US Citizen. ( OCI card available ) 3) Understand in both the above options we need to pay Stamp duty and Registration charges are involved. To reduce the cost can we have the transfer done at a nominal value of Rs. 1/ - 4) Also since my dad is very old to endure a 24 hour flight from US to India for in person registration at registration office would be difficult - What options would be available if he has to sign / execute from US ?