Who?

 Who? by Algis Budrys

Lucas Martino is a key scientist for The Allied National Government. An explosion happens when his doing his work and the Soviet International Block helps restore his health before restoring him. Many parts of his body are artificial. The allies are unsure if he is the "real person" or a spy. He seems to check all the boxes, but the allies are not sure. Eventually, they decide to dismiss him from his work leaving him to live a different life. It turns out that though the Soviets thought to replace him as a spy, they didn't. However, they did succeed in setting back the scientific work while still letting the many live.