When you're talking about a real life, non-fictional character, I don't think it's unreasonable to suggest that the actor should represent that person to some degree (inclusive of their sex / gender). It's not like we're getting artificially mad at Henry Cavill for playing Superman because Superman has always been a white man and we should instead give the opportunity to a trans black man because there are so few opportunities in film for trans black men. That would be silly.

Now, I don't think it's unreasonable for Scarlett to play the part,if that's what they want to do, but I also don't think it's unreasonable to suggest that you should consider a trans man to play the part of a character who was a trans man in real life.