Vehicle Buyer is not transferring in his name.

I sold my 2 wheeler vehicle (access 125) on 2017, I signed and provided all necessary document to the buyer. He promised to transfer the vehicles in his name. After then I headed for my higher education and back to the city after 3 years and found that still he did not transfer the vehicles in his name. Then again in 2021 I signed and gave the necessary documents to him and he said he will transfer the vehicles soon. In spite of multiple times conveying he gives some rubbish reason saying I am not in the city and he denies to transfer . Now he don't even pick my call and I don't have his residence details, he was just a colleague in my work place earlier. Now today I received the Legal Notice from RTO mentioning for challan payments for INR 2200, even though the vehicle is not with me and it is not transferred to the buyer name. Unfortunately I do not have the transfer document and vehicle original document that was signed by me and handed over to the buyer. All vehicles original document was handed over to the buyer. However I have the vehicles documents xerox(RC and buying documents and loan paper ) in place. What should be the next step that should i take now please advice. I made a blunder by not complaining to the RTO and Police station about the vehicle.