relatives from Ytteroy

My Gt grandfather Iver Nielssen (Brustad) was born on the island of Ytteroy - near Levanger. He left Norway to live in Swansea (UK). ONe of his brother, Sivert Zacahariason (Brustad/Kjolsvig) married Anna Jonetta in 1885 in Trondheim. They then emigrated to USA. Sivert's parents were Simon Nielssen Brustad and Margrethe Olsdatter bapt 1794/5. Where, I don't know. Another relative, Peter Olaf Gustav Olsen, son of Niels Peter Olsen and Anne Russine Nielsen (Brustad) also emigrated to USA. Peter was born on 31/07/1861 in Ytteroy. He emigrated on 06/09/1881. As I don't know where they sailed from or on which ship or where they settled, how do I go about tracing records for them in the USA. Many thanks