How to find all planetary positions on starting of every Yuga?
I am not able to be able to give you an answer with references because I do not think this question has agreement among seasoned Jyotishas. I can give you some hints on the difficulty of solving this problem. The key question is how to go about calculating the Ayanamsa. I know there are a lot of horoscopes of Sri Krishna and Sri Rama floating around on the internet and in books and expert astrologers will give discourses but the key issue to ask them is what ayanamsha to use ? Again there are horoscopes that talk about the beginning of Kali Yuga and again there is not clarity on what ayanamsa to use. Here is a very relevant article that talks about the vexed issue - Precession of equinoxes and a more modern perspective on it from Wikipedia - Axial precession
I used to use a very sophisticated vedic astrology software with a Swiss ephemeris but if I try to calculate the horoscope of Sri Krishna I find the birthday(which is published in the internet) to be in mid July. . To see the contradictions in this Rishi Vyaasa's Mahabharata says the birth of Sri Krishna took place in the Hindu lunisolar calendar month of Shraavana. Today this month is marked between July 15th - August 15th. Yet we celebrate today Sri Krishna's birthday every year in the month of Bhaadrapada(mid August - mid September).
Some of these ideas that I am espousing is mentioned in this outlook article - Sri Krishna's birthday.
The key question is - Is today's August Dwapara yug's July ? Have things been shifted ? To know this deeply a more than superficial understanding of the precession of the equinoxes is required.
So everything has changed because of the precession of the equinoxes. When we come up with accurate methods to accurately determine the ayanamsa (the present age of Kali is an inhibition in this path IMHO) then we can answer your question with great clarity.