An artist once suggested a weak solution of methylated spirit, dabbed on very gently with a lint-free cloth. I tried this and it did lighten the painting, bringing some areas to visibility. However the painting was already old and crazed so I wasn't too scared of damaging it - I would be reluctant to try this on something more valuable and agree you should have it professionally cleaned.
I'm an artist and I would suggest you have it professionally cleaned. Even though I paint and have many of my own works, I would NEVER attempt to clean them myself. You could destroy the surface and then the value of the painting would be less too. Hope this helps you out.
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