I think you could get away with any "later" flint style lock.
Since there is no existing Hawken, at least any known, undisputed examples, it would be tough for someone to say that your lock choices was either "incorrect" or "not possible".
Jake and Sam's father, Christian, was not exactly "brand loyal" when it came to locks - he seems to have used whatever he could get his hands on.
Likewise, Jake and Sam used quite a variety of cap locks on their (existing) rifles.
So based on both of those "knowns" regarding locks on original Hawken rifles, I would suggest you could use pretty much anything you wanted.
If you are leaning towards an English lock then the late Ketland (Chambers) by all accounts would be a first rate lock.
If you want a suggestion for a Germanic style, which (personal view) would have just as likely to have been used on a J&S Hawken flint, the Chambers Golden Age is a fine lock (own one myself).