Debbie,
If the info from Helmuth above is correct (his wife is an Isaak relative) then I think we have found your David Isaak! I have info on him, in my GEDCOM from my second cousin Fred Isaak, who has been to Germany to visit him (Helmuth) last year.
Helmuth states that he was married 3 times. I have info on his children, from his ?first wife Fredericka Maier. They had 9 children. I don't have info here on any further marriages. Fred's GEDCOM states a slightly different death for David (1957) but I don't know if that is the correct date of death (what did the Town History Book for Kulm, ND state?). Helmuth seems to think that it may be your date. If all is true, then I can tell you he is indeed from the Christian Isaak family that I mentioned. I have info on this family. Like I said, the singer Chris Isaak is related to them.
Christian Isaak is a cousin of two of my g-g-grandfathers. Both on my mother's and father's side. There is a biography of Christian Isaak is some book. I got it awhile back, not sure of the source.
I have info on the Isaak's back to Bessarabia, Russia.
If you have a genealolgy program that can open GEDCOM files, I will send you a file of the Christian Isaak family. My internet friend sent me a file of this family.
Hope this helps.
John Sailer