I don't believe Lee made this rifle.
Sounds like you have a pretty good idea of the things to rework. If it is at all possible, I would give consideration to making a new sideplate, that totally covers the area of the existing one, in a more appropriate style. I think the existing one started life as one of those small one-screw "Tennessee" style plates that TOW and some of the other suppliers sell, and then someone made a new one with the step and forward portion to cover more area. If you have not had it off, you might find that someone did this to cover up a mistake or a repair.
Also- if you are going to reshape the lock panels, do that first because the sideplate panel should closely match the same size - so that will dictate how much area you have to work with. Keep this in mind as you are shaping the lock panel, as you might have to "compromise" a bit between the 2 sides so that you don't take too much away from the sideplate panel.
Anyway, if you have access to some photos of pre-1800 longrifles, especially Lancaster style guns, you can get some ideas, but with the size you need to cover, I would say pick something close that works, regardless of whether it's Lancaster, York, Virginia, etc. Try enlarging photocopies and make yourself some paper patterns to lay on there and see what works. Try not to make the mortise for the sideplate any bigger than the biggest dimensions you have as your constraint now, which are the two (top and bottom) wide areas front of the bolt. It may not work - i.e. you may not have enough wood around the sideplate for it to work. But if you do, basically, make a new sideplate on which the area behind the lockbolt is more of the same size as the area in front of the sideplate, to balance it. Also - it looks to me like the front screw is a lot smaller than the rear and sits far forward, so maybe it actually is a woodscrew and not a lockbolt(?) You can probably fix this too, or leave it as a dummy bolt, your choice.
I would also reduce the width of that patchbox lid, if possible, and maybe the height/thickness (can' tell from the photos)
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However, see what's under that sideplate first and try to measure how much wood you will be removing from the lock panel before you try anything on the sideplate side.
These things are aesthetics, but I'd give the gun a good mechanical evaluation first too to be safe - breeching, bore, touch hole, lock, and triggers, make sure no dovetails got set too deep into the barrel etc. Sometimes aesthetic items can reflect the builder's level of knowledge and skill.
Good luck
Guy