What can I do about being attributed to someone who is not my biological father, and am I considered sinful if I keep my current name, taking into account the difficulty of changing official documents after 40 years and the death of all those involved, with no material evidence other than the aunt's testimony?

What is established by the authentic Sunnah (definition) is that a child is attributed to the one on whose bed he was born, for the Prophet, peace and blessings be upon him, said: “The child belongs to the bed.” Therefore, you are legally attributed to your father, and what your aunt claimed is of no consequence. The hadiths that condemn those who attribute themselves to other than their father do not apply to you. It is obligatory upon your aunt to repent to Allah and refrain from spreading such talk.