I subscribe to the idea that for a religion to compete, it needs some form of scripture.
The only "major" religions that survived well without some form of canon would be shinto and chinese folk religion, both of which only did so because buddhism was fortunately accommodating. Hinduism had the Vedas, buddhism innumerable sutras, Sikhism the Guru Granth Sahib (sorry to any Sikhs if I said that wrong) and I'm sure I need not say about the abrahamic faiths, or the incredible against the odds survival of the Parsi community.
In short, Scandinavian polytheism is going to need to establish a tradition of writing prior to its interactions with Christianity.
Then you get all sorts of issues though... Chances are you butterfly Christianity, the still dominant west having more to gain out of adopting a Germanic faith, but also you sorta butterfly Odin by going too far back. Traditionally it was Tyr who was king of the gods, so it arguably fails Odins challenge.