Filling a crack in a wall with roughcast on limestone
You'll need to pry, chip, scrape &amp; wire brush off any &amp; all loose, soft or mushy stucco, possibly tripling the hole's size. This has to be done if you don't want your patch falling out very or fairly soon after it's done.

But, any stucco, cement or mortar mix does &amp; will work perfectly. Buy a bag of small gravel &amp; insert the gravel into the wet cement product for a very close match to the surrounding pebble-like finish.

That's all I did 20-years ago in my first place &amp; it's still there with no signs of any problems. Additionally, any surface cracks should be finger force caulked with a white latex caulk to avoid more future holes.