The MLA Handbook, 8th ed., does not address how to handle stage directions. According to the 7th ed., writers should treat stage directions the same as any other quoted material within the quote and reproduce them exactly as they appear in the original source, unless they do not fit the context of the sentence. In that case, an ellipsis can replace the stage direction to keep the grammatical sense of your sentence.
The MLA8 handbook addresses quoting drama on p. 80.
