I believe that Toriyama didn't have on start in its mind the idea of Goku's origin being that of a saiyajin alien or even something similar, it doesn't match at all with the concepts shown in the first Dragon ball series.
I know it's based on Sun Wukong from Journey to the West, but I think it's just an inspiration for the character, not that the character origin was going to be the same than that character.
Has Toriyama ever stated what was his original idea for the origin of Goku? If so, which was it? Or didn't he think about that until the Raditz Saga?