Tooth-yellowing mostly caused by tea, coffee and tobacco smoke. Fortunately, most modern toothpastes contain a whitening agent.

I do not bother with de-yellowing any more than I would paint over a mediaeval chair. But for those who care, there is a vast online experience and correspondence in threads using the term "Retrobrite" or "Retrobright" (a TransAtlantic choice).

Plastics decay to brittleness is a much worse long-term threat to vintage computers. Protection from UV light is not enough, in the long run. I predict 3D printing of replacement plastic elements will become more significant in coming decades.

Rick