We killed a Yuan-ti. Now we want to infiltrate their base disguised as one of the Yuan-ti we killed.
The PHB Page 211, Alter Self spell states:
You assume a different form. When you cast the spell, choose one of the following options, the effects of which last for the duration of the spell. While the spell lasts, you can end one option as an action to gain the benefits of a different one.
Change Appearance. You transform your appearance. You decide what you look like, including your height, weight, facial features, sound of your voice, hair length, coloration, and distinguishing characteristics, if any. You can make yourself appear as a member of another race, though none of your statistics change. You also can’t appear as a creature of a different size than you, and your basic shape stays the same; if you’re bipedal, you can’t use this spell to become quadrupedal, for instance. At any time for the duration of the spell, you can use your action to change your appearance in this way again.
One would assume that Yuan-ti’s are a "different" form due to the snake torso.
Disguise Self is also too limited. Polymorph only covers beasts.
What can be done if, as a wizard, you are trying to disguise yourself as a particular Yuan-ti you met on an adventure (that is now dead)?