As others have said, you can do whatever you want with one of these factory rifles.
For instance, you can buy a nice silver or German Silver (not real silver) inlay from places like Dixie Gunworks or Track of the Wolf.
A big star or crescent moon always looks good on the stocks cheekpiece.
If you don't want to spend any money at all and you have a piece of contrasting wood, you can make your own inlay like the one shown on my Lyman Great Plains Rifle:
How fancy or modern you want it to be is up to you but I suggest trying to keep with the style used on the real guns.
I've seen some masterful carvings of elk and deer on some of these rifles stocks and the owners were quite proud of them.
To me, they seemed totally out of place though.
The antique rifles that have survived never had such pieces of art work on them.
Although it looks rather crappy to me, something that was often done was the owner would carve their initials into the stock.
This can be seen in the photo below. It is crude, but historically accurate.
Some folks have driven brass headed nails into their stocks to give the gun an "Indian look".
There is no doubt this was done in the past but be warned: If you have a beard, those tacks have a habit of grabbing your whiskers and yanking them out when the gun fires.
Other things that are commonly done is to strip the bluing off of the barrel and brown it.
Shiny brass can be polished or conversely, aged so it looks 150 years old. Whatever floats your boat, as they say.
Have fun. :thumbsup: