Make "Ain't" a word. I mean, it works. Ain't = Am+Not. The "I" comes from letter replacement that is frequent found.

It does fit. Look:

Ain't
Ain't = Am/Are+Not
It can be used to be a negative for words like I, We, They, et cetera. For example:

I ain't (am not) going to go to the movies.
We (are not) going to watch a movie.
They ain't (are not) avid movie-watchers.

Plus, since America is all about shortcuts, this works.