Have you considered having someone over paint it in the correct colors? Much easier and probably much safer (if sanding is involved) than trying to strip it.
I would contact a local (or with the internet) sorta local small museum. One of their curators may tell you who to contact for information on how or if it's even practical, to remove the paint.
BTW For painting on canvas, I use lead based paints for doing my historical portraits. The lead in lead white and naples yellow makes the paint stiff and quick drying with leads to a quicker build up of paint, to help get that 3-D look. And yes in 100 years it will yellow, which is why I don't use that white for contemporary paintings.
Mind you I'm not eating it, not fingerpainting, and not sanding it down...
Don't get me started on some of the other toxic pigments.