When you say you're the sole heir -- was there actually an estate probated? Or were you a joint owner of her assets, so a probate proceeding wasn't necessary? If it's the latter, an agent (or publisher) may require a formal proceeding before getting involved.
The laws relating to wills and inheritance vary from state to state, but the contract may not be enforceable after your mother's death.
I would recommend consulting with an attorney in your jurisdiction before proceeding with this. I don't know for sure what an agent would require, but you should find out (probably a free consultation, at least for starters) what it would take to be appointed the executor/administrator/representative of your mother's estate (or to establish, legally, that you are the sole heir, and therefore the holder of the copyrights), if an agent required you to have that authority.
As an aside -- "literary executor" is a term that's thrown around a lot, but is not well defined in the law. Unless there's a will that actually uses that term, I wouldn't recommend claiming its authority. If you're the sole heir, AND there's a probate proceeding that confirms that, OR there's a probate AND you're the executor/administrator or personal representative of the estate, then I would use whichever of those descriptions applies. (Some states still refer to executors as the person carries out the terms of a will, administrators as the person who manages an estate when there is no will. Other states are adopting a simpler procedure, whereby the person managing the estate, regardless of whether there's a will, is simply called the "personal representative." Either way, those positions are separate from a "literary executor" or "literary representative," who would be named and empowered in the will -- there must be a will, in this case -- much like a trustee would be.)
Not giving individual legal advice, just general information, and taking every opportunity to remind authors to discuss the post-mortem management of their copyrights with a legal professional in their jurisdiction, because it's actually a quite complicated issue.