Colonial gunsmiths scavenged any part they could get their hands on. They would have assembled these to a rough simplified "pattern" based on the common muskets and fusils at the time. Based on this, you would not go wrong getting a Northwest Trade Gun, provided it was not marked with the common trade markings of the later date (sitting fox, bow & arrow, etc.) Just replace the large bow triggerguard with a guard from an old French or British musket (you'll probably need to trim the long trigger down) and the sideplate with a "generic" pattern. Track of the Wolf has these parts in their catalog and the pictures are full scale, so you can tell roughly if the part will fit your trade gun. The triggerguards and serpent sideplates of today's commercially available NW trade guns are generally surface-mounted, so it should be a relatively easy swap. The trade guns were based on the muskets and fusils of the late 17th-early 18th century, so the architecture is generally correct.
Similarly, you could order a French trade gun or fusil de chasse and request the markings be left off. This would give you a colonial fusil made with scavenged French parts and built with French architecture, also common at the time.
If you already have a Bess, I suppose that removing the lock markings would yield a Colonial gun. The steel ramrod would be out of place; a wooden one would be more appropriate.
Just some thoughts, for what they're worth. I'd recommend "Battle Weapons of the American Revolution" and "Flintlock Fowlers" for their indispensible and profuse illustrations. Pore over those and you'll see where my suggestions are coming from.
