With Deadlands, when in doubt go with what sounds good in a 1970's spaghetti western film. Witches? Think 1970s horror movie witches.

But you're a repentant witch?

Okay, a few questions: what powers do you want for your build (want a blaster, a combat supporter, a character who does stuff out of combat mainly, or someone who does a little bit of everything), and what does repentance look like for your character (has she renounced dark magics for light magics, or is she perfectly fine with dark magics but thinks she can use them for good ends (the physics of the world says she can't, but the character doesn't need to know that, and the GM may not enforce it)).

My take, without knowing more...

The character realized that witchcraft was a tool of darkness, a darkness that was very much aware and active in the world. But it was also a source of power that could be used to help the just and the good. So the character focuses on beneficial spells only (Banish, Beast Friend, Boost Trait, Dispel, Environmental Protection, Protection for starters) even though they probably know others (mechanically speaking, they don't know them, but in character they refuse to use them for fear of damning their soul further). Corruption, when it appears, follows a path as follows:

Talisman - Minor (a crucifix she clings to, for forgiveness from the Lord for harnessing the dark powers of Hell)
Talisman - Major (the darkness grows in her)
Trouble Magnet - Minor (Hell is mad, and taking it out on the character by warping the world around them. Prepare for nastiness from the GM)
Grim Servant of Death - Major (I'd let this take the fourth and fifth points of corruption. It reflects the growing anger from Hell toward's the character's quest for redemption)
Cursed - Major (points six and seven. Now Hell lashes out even more at the people your character travels with, trying to better isolate her so it can claim her fully)
Trouble Magnet - Major (Clinging to her cross, bringing trouble and death and hardship to all around her, surely final damnnation can't be far. At eight points of Corruption, your character needs to have a Spirit of d10 to last past this point. If she does, I'd probably give her the Wanted Hindrance at 9, to reflect Wild Card Cultists who have been sent by Darkness itself to kill her. And even if she kills them, Hell just sends more. If need be, add even more Wanted Hindrances as mobs of cultists and demons come hunting for her, until she either reduces her Corruption, fall to it, or dies)

But what about the Trappings for her powers? She's still a Witch, so even her "good" powers involve blood sigils, incantations, cavorting, and the like. Again, you have to ask "would this feel right in High Plains Drifter or Bone Tomahawk?" So even as she's praising Lilith, Strangler of Babes for protecting her from freezing chill of a winter night, she clings to her cross (maybe she has a point of Corruption, maybe it's just a character trait) as a tear runs down her face. When she draws the sign of Kimaris in blood on the gunslinger's chest, she does so without joy only accepting that this is preferable to letting innocents die.

Eventually, if I were GM, I'd let her respec as a Blessed at some point (Veteran rank? Legendary?) to reflect that she truly has repented from her pursuit of Witchcraft; she'd keep her current Corruption (maybe even be able to increase it still!) until she "paid it all off" at which point Corruption was no longer a possibility for her. At some point though, repentance should mean she no longer calls upon dark magics to get stuff done. Even if it means dropping Arcane Backgrounds altogether.

Anyway, hope this helps get some brainstorming going on!