If Sigfrid was born out-of-wedlock, assume that his birth surname was that of his birth mother, PERSSON. If you lack his full copy birth certificate, apply for it to obtain his exact date of birth. The address of the Minnesota Dept. of Health is among the 50 states listed at :http://www.cdc.gov/nchs/howto/w2w/w2welcom.htm
When you get his exact date of birth, use it on the Advanced Search feature at RootsWeb's version of the online Social Security Death Index :http://ssdi.rootsweb.com
You can search with only a first name and date of birth, if you think his last name was changed by adoptive parents. We have no way of knowing if his first name was changed also, so we---I mean you---search on the assumption the first name was not changed.
I am reading a book by E. Wayne CARP, "Family Matters." I recall that Minnesota had an early law for confidentiality of adoption records, but I cannot quickly find the relevant page in the book. That could pose a problem for your research.