I didn't know someone had made a flat-chested Yasu sprite, but looking it up on Google now, you're probably talking about this pic -
http://malak1147.deviantart.com/art/Yasu-207106387 - which looks pretty good! Or maybe this one
http://joesca.tumblr.com/post/5535521476.
Good luck with asking the person who made the Yasu sprite, whoever they are. If they give you permission to use the sprite you're talking about, but aren't willing to go to the trouble of making a set of them, you might still be able to manage to paste on other expressions to it. That might not be too hard.