case of bike being transferred by a non registered owner

i hav purchased a bike from a person X(on seeing an advertisement on OLX by his BROTHER A) by paying all the amount and he gave all the papers including rc book, insurance, signed copies of transfer papers from thew original owner(from y) and a document saying that he is transferring the bike on the owners Y's behalf to me Z( given in writing by x) stating that he will give the left NOC FROM HDFC BANK within 6 days. Its been 3 months since he has been delaying the supply of NOC, on doubting i enquired from hdfc bank and found out that 8000/- was outstanding to be paid by Y still to bank. now the thing is X has ran away to another state and his brother A and Y(govt employee) are still in the city and still postponding the papers saying that there is a dispute in bank for payment made(Y says he has already paid the amount in full). NOW i want to know that what should i do as the person from whom i purchased is not available and i have no written document from original owner. what should i do please help?