FamilySearch has headright records of various states in digital and microfilm form.
Your document seems to indicate that the boys' natural mother was granted 50 acres of land by the colonial government in exchange for serving out a 7-year indenture for a colonial sponsor after he paid her overseas passage to come to the colonies. It is highly likely she was single when she made the journey.
That may mean the headright may be in her maiden name. I do not know whether the headright would be recorded upon creation of the indenture agreement or upon completion of the 7-year indenture.
If the latter, she may have been married by then. This, of course, would affect whether to find her by her married or maiden name -- in fact, I wonder whether both might be listed for clarity if the headright were granted at the end.
Hopefully it lists their "natural Mother's" name. Another source I checked indicated that headrights were not granted to indentured servants, but only to 1 colonists already in the territory, 2 colonists who paid the passage for indentured servants to immigrate, and 3 immigrants who paid their own passage.