Not from gunsmithing point of view, but if there is a large cost involved, either from a dollar point of view, or from a personal time and resources viewpoint, if the gun ever was sold, the return might be less than the cost.

If the plan is to have a personal gun to enjoy, that might not be relevant.

The Greener Empire side by side shotguns were used to make some double rifles out of. Not bad shotguns in themselves, but not expensive if the plain models. When made into a .577 double rifle, or a cape gun in .577/12g they realised about A$4500 to A$6000 on the resale market I believe. One of our members has one, can't remember the price it sold for, from one member to another.

Another plan can be to try to purchase a 12-bore double rifle. If a plainer model without much engraving the price may not be too expensive. Probably not that easy to source and would take some time.

But again if doing a lot of work yourself, and for a personal fun gun, the above might not be relevant at all.