https://en.wikipedia.org/wiki/Thinking_man%27s/woman%27s_crumpet According to this link the expression refers to someone who is appealing to the opposite sex because of their intelligence as well as their good looks. That is, the word 'thinking' refers to the object of desire, not the person who is attracted to them. So I don't think you can substitute 'working' or 'farming' in this expression. By the way, the term itself is not particularly politically correct these days, even though it is supposed to be humorous. I don't think there are many women who would like to be labelled as someone's 'crumpet', but maybe that's just my opinion.