I did a screen shot of your picture showing how you have begun to reshape the lock panel(s), printed it and drawn in some suggestions. This is definitely not the last word or the only way t approach this work, but it is what I would do.
I have drawn, as best I could, around the lock plate and your outline. You will see that me lock panel line is between those two lines. First you will see I have drawn a centre line through the wrist and the tail of the lock...the back end of the panel should intersect that centre line if possible. Also, it can have a little bit of a radius rather than a point, and that will soften the effect of meeting the centre line. Along the bottom, I have cut that part of the panel in half more or less. The bottom of the stock should have a flat area along each side of the trigger plate about 1/8" wide, and from that point up to the lock panel, the stock is almost flat. Don't worry about what may seem to be a lot of wood under the lock, between the bottom line of the lock panel and the flat along the trigger plate...many of the originals did as well.
Now, the forward end of the panel. there are two approaches you can use here: first, as in this first picture, the top of the lock plate can be beveled up to the edge that meets the barrel where it still remains parallel to the bore or the flat lines of the barrel. If you check out the upper line of the panel behind the hammer and follow it forward through the snail of the breech, it flows right into the panel forward of the snail, albeit dropping a bit. The other approach is to cut a quick radius in front of the lock plate as most other muzzle loading rifles are done, then finishing as I have drawn and is very similar from that point on with the first scenario.
When cutting the edges of the panel, especially at the tail end, don't be tempted to accentuate the concave edge with a rat tailed file. Just let it flow into the wrist. The forward end is more accentuated. By no means cut a concave cove along the top or bottom lines of the panel. Rather, a slight roundness or convex shape is desirable.
This particular rifle has one of the nicest shaped hammers available on modern 'copies' of Hawken rifles. And the triggers and guard are excellent as well.
A little more description of the top lines of the lock panel...As you follow the top of the panel behind the hammer moving the eyes forward, you will notice that that top line, as its terminus, aligns with the top edge of the side flat of the barrel. Likewise, if you follow the top edge of the lock plate behind the hammer forward through the snail, it aligns with the top line of the panel forward of the snail. This subtle sculpting is one of the features that consciously or otherwise, endears the Hawken rifle to us. Miss it, and you will not doing all you can to capture this rifle's grace.