I think "amn't" should be a word.

Consider the following:

You are not == you're not == you aren't
He is not   == he's not   == he isn't
I am not    == I'm not    ==  ???

This is a gross miscarriage of justice.

Well there’s ‘Aint’.

“I aint”

It is a word.

Well, that was anticlimactic.

I say “aren’t”, but only in the inverted form, e.g.:

I’m not going today.
Aren’t I going today?

And if you want to spell that “a’n’t” instead of “aren’t”, that fine by me.