The polite version would simply be 元気がありませんな.
The verb in this case is ある, ない is the plain form of the non-past negative ありません.
な at the end of a sentence is an introspective particle, it's used when you're talking to yourself. Sort of like "Not looking well, huh?". But unlike ね which is sometimes translated as "huh?" or "right?", な doesn't actually seek agreement with others, and rather indicates a comment to one's self.
じゃない can not be used here because じゃない is the plain form of じゃありません which is used to negate nouns and as the negative endings of な adjectives.