This is just a suggestion, but here's what I'd do:
Visit Bill Marx's site 'sonofharpospeaks.com', click the 'contact' tab and send him a note asking about the painting. If there's a person alive that might know anything at all about it, my money would be on Bill Marx. I contacted him myself a while ago and he sent me a very friendly reply. Worth a shot I think.
Is the painting a version of Harpo himself dressed as "The Blue Boy?" Is it an original with a signature? Your best bet is to check with an auction house that deals in celebrity artwork, and Harpo's family for authentication. I own a Harpo original of an adobe village dated 1961.
Best wishes, Pat