It's possible that an old acquaintance from Hogwarts who has family ties in America could assist him. This person might already have a network in America and could introduce Harry to important people. Also, there could be some reclusive magical scholars in America who, out of curiosity about Harry's powers and his role in the UK, decide to help him. They could teach him new magical skills that are specific to the American wizarding world.
His friends from Hogwarts, like Ron and Hermione, are likely to help. Ron could bring his family's practical knowledge of wizarding homes and Hermione would contribute with her extensive research skills.
One possible plot could be that Harry, tired of the constant danger and scrutiny in the UK wizarding world, decides to flee to America. He might encounter new magical creatures unique to America, like the Thunderbird. He could also learn about different forms of magic practiced there, such as Native American magic. And perhaps he'll make new friends who help him hide from those who might be looking for him in the UK.
Dumbledore might offer some guidance. He has vast knowledge and experience. He could tell Harry about the significance of different elements in a staff and how to combine them to make a truly powerful and unique magical tool.
Dumbledore might help in a spiritual or advisory way. Even though he's passed away, his wisdom could be passed on through memories or some magical means. Also, Hagrid could be a great help. He could assist in building areas suitable for magical creatures with his love and understanding of them.
His best friends Hermione and Ron would surely be there to help. Hermione with her vast knowledge of magic could research about the egg, and Ron with his loyalty would stand by Harry's side in any danger related to the egg.
Ron might be there too. He could use his skills in handling magical creatures to populate the island with friendly ones. And he could also help with fishing in the surrounding waters using magic, providing food for those on the island.
Hermione would most likely help. She's always been there for Harry, and with her vast knowledge from books, she could offer a great deal of advice on how to take care of the dragon egg. Ron might also be involved. Despite his initial fear of things, he has a brave heart and would probably stand by Harry's side.
America can help in Harry Potter fanfiction by providing additional manpower. Imagine a situation where Voldemort's influence spreads globally. American wizards, with their own set of skills and abilities, could join forces with the British wizards. They might have different approaches to combat magic, like more emphasis on elemental magic which could be a new strategy against the dark wizards. Also, their magical creatures might be different, and some of them could be used to counter the threats in the story.
Maybe Hagrid could help. He has a great love for magical creatures and might know of some special materials or spells that could be used in boat - building. He could also provide the physical strength needed to move heavy parts of the boat during construction. Also, Neville could be involved. He has grown a lot in his magical abilities and might contribute some unique plant - based magic to the boat - building process, like using magical vines to hold the boat together.
Dumbledore, if he were still alive in the fanfiction, could offer his wisdom. He might guide Harry on the moral and magical implications of creating an island. He could also provide some powerful spells or artifacts that would aid in the construction process.