Agree with Zonie that the "conventional wisdom" tends to say that if a Plains style Hawken was built in flint that it would "most probably" have a lock similar to the Late English by L&R.
I will give you a slightly different perspective that you can do with what you want.
There are a few Sam Hawken flintlock's still kicking around that existed from his days of working in Maryland and Ohio before he went out to St. Louis.
There is also a good number of sub-50 cal rifles that Sam made after Jake's death - 36, 38 and 40 caliber seems to be the most prolific that survived but there were also 42, 43 and 45 calibers.
Both the brothers learned gun making from their father and he wasn't too picky about "what lock" went on his rifles. It might be that they were of the same mind until they finally started producing their own locks - in fact, they did use locks from about a dozen different makers on their cap guns.
Here is an early S. Hawken Flint. No, doesn't look like a St. Louis gun but this is what he was making early on.
Now the pic isn't great but I think you could use a Ketland, an Egg, a Twigg or even something like a Chambers Golden Age (which I used on a C. Hawken build and barely had to touch the shape with a file to match a lock Sam's father used on an original).
It kinda depends if you want to make a "flintlock version" of a Plains capgun or you want to make it look like it might have actually looked like if was built near the end of the flint era - not the same thing..