Thor’s hammer, Mjolnir, and its counterpart, Stormbreaker, are among the most powerful weapons in the Marvel Universe, but even they aren’t indestructible. Fortunately, if the magic hammers ever do break, there is one hero who has proven he has the power to repair them: Silver Surfer.
Created by Jack Kirby, the Silver Surfer made his debut in The Fantastic Four #48 back in 1966. Before he began surfing the cosmos, Silver Surfer was an astronomer named Norrin Radd on the planet Zenn-La, which one day became targeted by Galactus, the devourer of worlds. In order to save his planet, Norrin offers to serve Galactus as his herald in exchange for the world-eater sparing his home world. Galactus accepts and imbues Norrin with some of the Power Cosmic, an incredibly powerful source of energy that grants its users god-like powers such as teleportation and matter transmutation. With the Power Cosmic, Norrin becomes the Silver Surfer and works to find planets for Galactus to consume - that is, until he finds Earth and manages to free himself from Galactus’ control with the help of some other Marvel superheroes. However, despite cutting ties with Galactus, Norrin keeps the Power Cosmic, which he later uses to repair the legendary Stormbreaker.
Stormbreaker may be Thor’s new weapon in the MCU, but in the comics, it's actually wielded by Beta Ray Bill, a rival-turned-ally of the God of Thunder. Beta Ray Bill is one of the few others in the Marvel Universe worthy of Mjolnir, so after Bill saved Thor’s life, Odin had the same dwarves who created Mjolnir craft Stormbreaker for Bill, and he’s been a friend to Asgard ever since. So in Cosmic Powers Unlimited #5 by Greg Wright, Asgard is where Bill is taken after he is injured in combat. To save Bill from certain death, Odin and Silver Surfer combine their powers to heal him, but Norrin goes a step further and entirely remakes the damaged Stormbreaker using the Power Cosmic, giving Bill a new and improved version of the weapon when he wakes up. Even Odin is shocked by Silver Surfer’s hammer-mending abilities, and shocking the All-Father is no easy feat.
As mentioned before, the same dwarves who created Mjolnir also created Stormbreaker; they even used the same metal, called Uru, for both weapons. Uru is an incredibly rare, durable, stone-like metal with a natural affinity to magic, making it especially responsive to enchantments. This may be why Silver Surfer was able to imbue Stormbreaker with the Power Cosmic and repair it. Regardless, since Stormbreaker and Mjolnir are practically twins, it stands to reason that Norrin would be able to repair Mjolnir in the same fashion should the need ever arise. Its a shame the Silver Surfer isn’t already in the MCU - Thor sure could have used his help when Hela broke his hammer in Ragnarok.
Odin may have helped with healing Beta Ray Bill, but judging by his surprise at the remaking of Stormbreaker, it’s safe to say the weapon upgrade was all Silver Surfer’s doing. You don’t mess with the Power Cosmic.