Almost no chance whatsoever.
For one thing, the CIA just wasn't very good at direct assassinations. They certainly tried: there was an attempt in 1960 at Patrice Lumumba with poisoned toothpaste (the station chief for the Congo refused permission to go ahead with the plan) and the famous attempts at Fidel Castro (the sheer number of attempts is overblown, but not the comedic quality to the ideas that were thrown around). The closest to a direct CIA assassination was Che Guevara, but even that was technically bungled, as the orders were to capture, not kill (*). Success was generally through support of groups who eventually did the deed; in the case of Lumumba (who died January 1961), the assassination group was led by Belgians.
The CIA's forte was bribery. They could throw cash at the right people and have entire governments in their pocket (I've written about this in the case of Jordan, where King Hussein was paid by the CIA for two decades; in more recent times it has come out the CIA had bribed the aides to President Karzai in Afghanistan). In the case of space scientists, given the restrictions the scientists worked in, and the US's desire for the knowledge they possessed, the greater concern for the USSR was defection.
The highest-profile defection of relevance is Colonel Grigori Tokaty, chief of the USSR rocket program at the end of WWII. In 1947 he was in East Germany helping give advice on aeronautical bombers when was suddenly recalled (his dealing with foreigners had led to suspicion he was a spy); he learned that he was about to be arrested and crossed the border to West Berlin. (He had knowledge of Soviet ballistic missile capabilities and was later described as a "godsend" to Western intelligence.)
The "assassination" being the concern is induced from comments like this one of Khruschev's (July 9, 1958, East Germany)...
In due time the photographs and names of those glorious people will be widely known by the people. We highly value these people, we prize them and protect their safety from hostile agents who might be sent in to destroy these outstanding people, our valuable cadres. But now, in view of the safety of the country and of the lives of those scientists, engineers, technicians, and other specialists, we may not as yet make public their names and their photographs.
...but it hopefully goes without saying that official government statements aren't always fully telling the truth.
The USSR space program was always a military one, with secrecy akin to that of the atom bomb. In international space meetings, Korolev and other high-level people of the engineering part of the space program couldn't come, so the USSR sent scientists or engineers who only talked about science instead, often with meaningless badges like "Intercosmos". In a book about Korolev, Hartford recounts
Americans formed friendships with some of the technical people, who must hae been delighted to have gotten through the KGB screening process that allowed them to qualify for attendance. It was virtually impossible, though, to get technological, as contrasted with scientific, information out of these wary people. One could sense their discomfort when questioned about space projects that the USSR was undertaking, or rumored to be undertaking.
The space medicine expert Oleg Gazenko stated in 1994:
Between 1950 and 1960 we weren't allowed to publish anything. We could not even use our own names on committees.
When Garagrin became the first man in space on April 12, 1961, Korolev didn't let any journalists onto the launch site; after the launch was over, he did a filming session that was carefully edited to remove imprefection (i.e. if a name was mispronounced, it was cut out). The record was purely for internal use (and later historical posterity).
The Soviet concern about leaks was extreme. The cosmonaut Sevastyanov wrote a doctoral thesis about a two-man shuttle which Korolev read and wrote on it "To keep for eternity". The thesis was classified and then destroyed.
To get back to the main question: the CIA didn't assassinate people as much as dump money and support into the laps of groups with grudges. There was no militant rebel group aiming to assassinate space scientists, so that approach wouldn't have worked. Additionally, the famous paranoia of the USSR brought secrecy up to such an extent that even their own contention that researcher names were not disclosed for "safety" is dubious, and if they had any realistic concerns, it was researchers defecting to the West, not assassination.
...
(*) Félix Rodríguez, CIA officer, was present at Guevara's execution. He was under orders to "do everything possible to keep him alive" but gave the order for execution and specified to try to make the wounds look related to combat. More on this at the National Security Archive.
Gerovitch, S. (2011). "Why Are We Telling Lies?" The Creation of Soviet Space History Myths. Russian Review, 70(3), 460-484.
Hartford, J. (1999). Korolev: How One Man Masterminded the Soviet Drive to Beat America to the Moon. Wiley.
O'daly, K. (2016). Living in the shadow: Britain and the USSR’s nuclear weapon delivery systems 1949-62 (Doctoral dissertation, University of Westminster).