'Okay' in the 17th century, which I guess is the feeling you want to evoke to the story, I think it doesn't fit.
If you want the reader to feel like it's the 17th century, you have to offer him as much stuff that makes him think that he's in the 17th century. Language would certainly show him that.
"Okay" would just throw him out of there and put him on a 21st century street.
"All right" or "Alright", is what I thought of first.
"Then so be it" or just "so be it" , was another.
"Fine" ...even though, to me, it feels modern, I dunno why. perhaps adding "sir" or "miss" after it feels more antiquated. Although I have to say that "Fine", seems to present a state of being as in "I'm fine".
"Certainly" was another.
*nods* ....does it have to be a word?