Kill the shark and get the head back.
Something we learn from the Silmarillion: when you're on a quest to "go get the thing, and don't come back without it", and the thing gets eaten by a monster, you can show up empty-handed, and hope your patron respects your effort. Or you can hunt down the monster and cut the thing out of its stomach, like a boss.
So get a harpoon and a bigger boat and go to work.
I mention this option first because it's probably the most fun.
You also mention the option of resurrecting the pirate using the remaining parts, but it's been suggested he wouldn't consent to that. I wouldn't be so quick to assume. He might not come back just so you can decapitate him, but what if you offered to turn him in alive? Even if he's facing a death sentence, if you can get him a blessing or burial rites or whatever else priests do in your culture to guide people into the afterlife, it may well be a better deal than he's getting now. And there's always the slim chance he could escape.
You'd need to communicate this offer to him somehow, but you've got a 13th-level necromancer, right?
All right, you really want to create a replacement head.
An issue with this plan is that any method for replicating a head you used to have is also a method for replicating a head that's still attached to its owner. If your patron isn't stupid, they must be aware of the risk of receiving a substitute head (most likely from the pirate himself, who would have the most incentive to fake his death as well as access to the real head). In particular, speak with dead is a simple way to authenticate the head and will really ruin your day.
However, if you're set on trying this plan, the go-to option is the 4th-level spell fabricate:
Choose raw materials that you can see within range. You can fabricate a Large or smaller object (contained within a 10-foot cube, or eight connected 5-foot cubes), given a sufficient quantity of raw material. If you are working with metal, stone, or another mineral substance, however, the fabricated object can be no larger than Medium (contained within a single 5-foot cube). The quality of objects made by the spell is commensurate with the quality of the raw materials.
Note that a severed head is an object, not a creature.
To produce finely crafted items, you have to have an appropriate craft proficiency, and I don't think there is one here, other than maybe Medicine. On the other hand, one can hardly expect a pirate's head to be delivered in mint condition.
As for raw materials, a pig carcass should serve, though you might prefer to use the body parts you do have, just in case they do a DNA test or something.