I suggested a while back that the alchemy rules in Knave 2e could serve as a template for creating other kinds of magic items. Let’s explore that in a bit more detail here. As a reminder, the basic alchemy formula from the rules goes as follows: ingredients (monster parts or rare substances) + cauldron + time (a watch or more) + INT roll. This process allows for the creation of a potion, a single-use magic item. My argument is that this basic formula can be replicated elsewhere. Let’s think through what this might look like below.
By the Power of Greyskull
Forging melee weapons should be a function of STR, since that’s the ability used to make melee attacks. Also, STR doesn’t have many secondary uses in the ruleset (unlike INT and CHA), so giving something else to this ability provides a bit of balance. Let’s keep the ingredients and time requirements and require an anvil instead of a cauldron. Both are too big and bulky to carry around in a dungeon. We’ll keep the price of failure, too. You can lose your weapon to a poor attempt to make it better.
As for what you can do with enchanting a weapon, the same exceptions-based approach used in alchemy can work here. Whatever rare ingredients you chopped off that thing with all the spiky bits can provide a clue to what you want to add to your sword. Discuss it with the GM, make the roll, and see what you get. A couple of exceptions I can see are repairing broken weapons and upgrading weapons. Both seem reasonable, but should probably be handled differently.
I like the idea of repair as requiring the same ingredients used to make the weapon originally. This might require a certain specific quest, and that’s fine by me: it just gives the players more adventuring options. I think it’s also possible to ‘reforge’ the weapon with other ingredients, but this would mean losing the original power of the weapon. It’s a trade-off.
Upgrading is a slightly different case and requires me to describe one of my house rules, so bear with me a moment. While Knave doesn’t explicitly state anything about magic weapons, it seems to suggest that the only additions to a die roll should stem from either an ability or from a circumstance bonus. The standard +1 weapon of fantasy gaming doesn’t fit into this scheme, so when I use modules from other systems, I’ve been converting those pluses to a change in die size for damage. Thus, a one-handed melee weapon with a +1 enchantment translates to an upgrade from a d6 to a d8 damage in my game.
I think it’s perfectly reasonable to try to enhance a weapon this way using the same formula. I require that the remains of an undead creature be used in this particular case, since you’re associating a weapon more with death, but that’s just flavor. I do think you should make this kind of enchantment require a particular kind of ingredient, though. Again, it’s all about interesting choices: what kind of beastie are we hunting today?
The Silver Bow
Ranged weapons can follow the same formula, but switch STR for WIS. As for required tools, you could just require a bowyer’s (or a gunsmith’s) kit for the work. The important thing is that the work can’t be done in the dungeon. When it comes to ingredients, I might further stipulate that ranged weapons (being advanced technology) require monster parts from constructs or giants for their creation, but again, I find myself doing world building rather than design.
The Dreaded Factory
A lot of what GMs worry about with magic weapons is overpowering the characters. In my experience running Knave, this is not something you really have to worry about. Knave characters don’t get in a lot of fights, and when they do, losing you weapon is a possibility with every roll. This means that even a powerful weapon could be lost at any moment and repairing that weapon risks losing it forever. Further, any enchantment requires going into the dungeon or wilderness to acquire more monster parts and there’s a real chance those parts spoil on the way back to town. The work required and the possibility of loss involved make an ‘enchanted weapons factory’ unlikely without a lot of creativity on the players’ part.
And hey, if they manage to set up some kind of crazy scheme, that means they’re fully invested in your game. So you still win.
And fires are always happening in factories. It’s a real shame.

Stealing these ideas immediately. I had a hard time getting my players to engage in the alchemy rules, but if I pitch it as “DIY Magic Blacksmithing” I bet they’ll get on board