While working on Prodigium Reforged I had trouble finding a mod to overwrite or add repair ingredients of some items/gear that could be compatible with other mods (for example, being detected as a new reforge item for the Quality Equipment mod)

So if you're looking to fix this specific pain or just want to change or add the repair ingredient of any items, this should be able to help you.

How it works
The mod installs Mixin injectors at HEAD of isValidRepairItem on five vanilla classes: Item, TieredItem, ArmorItem, ElytraItem, ShieldItem. When an item has a registered override, the Mixin returns the result of testing the candidate against your custom Ingredient, replacing the vanilla check entirely.

How to use
Everything is done through datapack. It supports adding multiple items or even tag as a Repair Item.
data/<your_namespace>/repair_overrides/<anything>.json

{ "item": "minecraft:diamond_pickaxe", "ingredient": { "item": "minecraft:gold_ingot" } }

Multiple acceptable materials:

{ "item": "minecraft:trident", "ingredient": [ { "item": "minecraft:prismarine_shard" }, { "item": "minecraft:nautilus_shell" } ] }

Tag-based (any item in the tag works):

{ "item": "minecraft:elytra", "ingredient": { "tag": "forge:leather" } }

Known edge case
Items that extend Item and override isValidRepairItem in their own subclass aren't covered automatically (similar to how vanilla ElytraItem and ShieldItem needed their own dedicated Mixins). Report it on the issue tracker and a per-class Mixin can be added.