For plastic key-tops, I will use Scrubbing-Bubbles initially and followed up with some denatured alcohol if some stubborn staining remains.
If the plastic has "dulled" with age/oxidation...I will use Filtz metal & plastic polish on q-tips and rubbing out with soft cotton rags (small pieces to get in between the side edges.
I take them all off, stick them in a bowl of warm, soapy water, then clean them each with a toothbrush. After that, I'll Flitz them and pop them back on the machine top to bottom. Phil Forrest