When Roger Chillingworth first appears in Chapter 3 of The Scarlet Letter he emerges from the woods wearing a mixture of Old World and New World raiment. This is due to his time spent with the Native Americans since his voyage to the colony. Hawthorne says of Chillingworth
By the Indian's side, and evidently sustaining a companionship with him, stood a white man, clad in a strange disarray of civilised and savage costume.
If I were to dress like him, I would wear basic black pants and a shirt similar to what you see kids wear in Thanksgiving programs. Then I would make them tattered and add fake fur to make it seem like I had killed my jacket. This would give the look of both colonial and Native American.
Since the novel talks about his knowledge of plants, I'd probably carry some leafy plants with me to pretend I was making a potion or other remedy.