There is no Sivert Olsen Morseth from Singsås. I am sure that emigration record showing Sivert Olsen is actually your Sivert Svendsen Morseth.
The 1900 census for Lincoln county, Minnesota, says that Sivert Morseth was born in Sep 1854. This matches the birth record for Sivert Svendsen.
The 1910 and 1920 census records for Lincoln county both list him as "Sivert S. Morseth". The "S" is surely for "Svendsen".
Also, the death record for Sivert S. Morseth, dated 31 Oct 1920, confirms the birth date of 10 Sep 1854. It also names his parents as "Sven Morseth and Kari Kristiansdatter".