What you are thinking of doing is simply called an 'Upgrade' by the gunsmithing/restoration folks.
People like factory engraved, factory stocked and checkered, & orig finishes.
But not everyone can afford an original specimen of their favorite firearm in the higher grades.
So,,people build or have built an 'Upgrade'. Using a factory gun in less than collectible condition (usually orig finish is gone), they have the gun
engraved, blued, plated, stocked, checkered,,what ever to the old factory specs and appearance.
You do have to watch that your Upgrade doesn't end up costing more than what you may be able to buy an orig for. The work is expensive of course.
The practice is common as mud with Colt revolvers and semiautos, vintage USA mfg SxS shotguns like high grade AH Fox, LCSmith, Parker, etc.
The long guns get pricey as there is re-stocking involved with fancy wood that drives the cost way up.
The 'base gun' can also be tough to locate at times if wanting to get specific bbl length, caliber, and with SxS's , factory single trigger, ejectors, etc.
Same with Rifles, Winchester is a prime candidate to take one of their lever actions and restock, maybe convert to pistol grip deluxe. Add or already having things like 1/2 magazine, oct bbl, etc.
Engraving patterns are usally straight factory patterns from the old mfg'rs specs. They can be copied to a T, or get into a bit of custom work and layouts changed slightly to include more coverage for example. Or perhaps adding inlayed figures and such.,
Handguns generally much less of an investment to have made as the entire package is smaller obviously. But there may be grip making involved and usually is, there isn't the big expense of an imported walnut fancy stock blank and turning that into a butt stock and forend. Then checkering both to factory pattern(s).
Since you would be making a 'copy' of a factory produced gun, the upfront thing to add to it is some form of marking(s) that indicate it is NOT a factory original.
Usually just the gun itself will present itself as a non-factory item to most collectors. But there are some that excede that and have been passed off as original.
Mark it and it doesn't have to be a BillBoard, mark it more than one location is my suggestion. Though that can and has been overcome b y some less than upfront individuals.
Also make some small diviation from the factory specimen so as to note their difference.
There are plenty of Upgrades around in the gun world.
Unfortunately many have already been passed off to other buyers as originals.
People are just too hungry for the profit they may be able to make to not try it.