Consider this conversation:
— Iceland has more than 200 rainy days per year.
— Are you sure?
— No, I am not sure.
Is it valid or wrong English to say,
— No, sure I am not.
...in the last sentence?
I am German native speaker and in German both versions ("Nein, ich bin nicht sicher"/"Nein, sicher bin ich nicht") would be valid. The latter version is probably even preferred because that puts emphasis on the sure, as in "I do think that's how it is, but I don't know for a fact".
(It's impossible though to use the second version in an affirmation in German "Ja, sicher bin ich." This sounds like Yoda-speak, much like the English equivalent "Yes, sure I am.")
If I would know the grammar of my own language better I could have asked: "Is (sentence) inversion allowed in English grammar?". The second German version above falls under the rule of (Sentence) Inversion and the English translations in the linked article and all answers to my question indicate that such an inversion is not allowed in the English language.