The negative tag is always "aren't I". A literary way would be "Am I not?" ...but NEVER "amn't", not even in slang.

'ain't' would work, but it is VERY poor English.

What about "ain't"? That's slang and works.

I have to agree with Eliot on this one

No, "amn't" doesn't work for a very simple reason. Try saying "amn't" in every possible situation where you think it would fit. Observe how tiresome it is to do so each time. ;)