In his poem, Whitman goes through and lists many different types of people at work, singing their songs happily. This is a representation of how he felt that America was a beautiful place, and how each laborer was beautiful and happy in their jobs, and the singing is representative of their unique contributions, voices and talents that add to the general state of the country.
In his poem, Whitman lists professions that are heavily oriented towards manual labor (carpenters, mechanics, boatmen, shoemakers, hatters, ploughboys); if he were to write it today, he might want to diversify some other types of labor that have come through the specialization of trades, and the advent of technology. There are so many more professions today, so he could greatly expand his list of trades to write about. Another thing to consider is the nation's atmosphere, and whether Whitman would feel the same awe and wonder at its working people. Do we express such happiness and contentment in life? Or are people in America today more disgruntled and cynical? Could Whitman ask the average worker, "Are you happy with what you do every day?" and get an answer that would fit the spirit of his poem? That is an important facet to consider.
There are many different answers to this question; considering the advancement of trades, and America's changing attitudes are two important dimensions to consider. Good luck!