That is very cool and the right museum, if they had an existing collection on the Expressmen, would treasure it. However, I think that donating it to a museum that does not have an Expressman theme, it would end up gathering dust in their vault and possibly being deaccessioned (sold off to support the museum). So if you did choose to donate it, I would seek out a museum with an interest in the Expressmen.
I agree, don't clean it, leave it just as it is, it's terrific!