Ole Amundson 1866 - ????

I have been searching for my great-great grandfathers gravesite but I don't know when or where he died. He was born in Feb 16, 1866 in Kvam, Oppland, Norway. Ole moved to MInnesota, USA in 1867 with his parents Amund Olsen (1832-1902) and Mary Olsen (1842-1917). They immigrated on the Bark Rhea June 24 1867. Amund and Mary are both buried at Zion Lutheran Cemetary outside Watson, MN along with a couple of Ole's siblings. In 1892, Ole married Marie Rosendahl...I can't find out her date of death either. There is no one still alive in my family that can answer these questions and I've been looking for years and these 2 are the toughest to find. Both of them apepar in the 1930 US census in North Dakota but neither one appear in the 1940 census or at least I can't find them. There is an Ole and Marie buried in Mille Lacs county, MN and that is not them. Can anyone help? I'm so stuck. Thanks!!!