Well, Lohan's public persona is disgusting, for sure. But she is a very good actress and can sing well (kinda like Stevie). Lohan probably would do a great job with the part if she could drop the drugs and get to the movie set on time. But I don't think any director would trust her to do that.
And I love Megan, but I don't see her in this part at all. Megan should cut an album for herself in her own style.