Need some ideas

I am looking for information to help me trace my Norwegian roots. Here is what I have, maybe you can tell me where to look next. My ggrandmother Clara Amelia Tapio was born in Kristiana, Norway. June 22, 1887. Her parents were Carl (July 1859, Norway) and Hilda (Sept 1865, Finland). They left Norway a few weeks after my ggrandmother was born. So I know they came over in 1887. Here is my biggest problem.....They changed their name when they came over. In Norway, it was Johnsen (sp?). They changed it to Tapio when they emmigrated. I know they moved to minnesota. So where do I go from here? I might have found my ggrandfather in the 1875 Norway Census but I am not sure. SInce his first born son was named Christian, and the father's name listed in the census is Christian. I highly suspect that I found the right family but I am not sure. Any help or ideas are greatly appreciated. Thanks, Steph