Kloppen Family of South Dakota

I am researching grandfather and his forebears, the Kloppens of South Dakota. My grandfather's name was Severt/Sivert/Syvert Kloppen. His father was Kundt/Knut Kloppen or Kloppene. The family immigrated to South Dakota at the end of the 19th century. I cannot locate the name of my grandfather's mother or any other information about her. She died when my grandfather was very young. He was born in South Dakota. His siblings were Annie Kloppen Mathison and Hans Kloppen, also of South Dakota. There is little about individuals named Kloppen on Ancestry.com, where I have been working. I will be grateful for any information or suggestions about other sources to research.