Are there budget replacements for Ugin and Karn removal abilities?

Question

I am building a mono green Tron deck with a 60$ budget and looking for cheaper alternatives to replace planeswalkers listed below:

1. Ugin, the Spirit Dragon

2. Karn Liberated

I need a replacement for Ugin's -X ability because its kinda hard to deal with humans, spirits and similar creature spam decks. I also need a replacement for Karn's -3 ability which kinda helps a lot in mirror matches or when you need to take their combo piece away without them having a chance to get it back.

Answer 1

Obviously these two cards are unique in their capabilities and you should expect a decreased power level without them. However, there are cards that can fit the roles that these two planeswalkers fill even if they won't be quite as good.

World Breaker is a good choice to replace Karn as it is removal as well as a threat. It's a bit tougher to cast turn 3, but it also provides recursive removal which may help the U/W match-up.

Spot Removal

Ugin has a Lightning Bolt and Karn can exile a permanent. The following cards can help fill this role.

• Spine of Ish Sah: Like "Karn, -3". Potentially recursive.
• Walking Ballista: Threat + Removal
• Warping Wail: Limited removal with some flexible utility.
• Scour from Existence: A colorless Instant, not as good as Spine can be.
• Titan's Presence: Possibly viable in Eldrazi Tron.
• Brittle Effigy: Creature spot removal for 5.
• Universal Solvent: Poor-mans Spine.

Mass Removal

Ugins -X can destroy multiple permanents, so we need something to cover that as well.

• Oblivion Stone: Probably already in the deck, but could increase to 4.
• Ratchet Bomb: Excellent against tokens, configurable destruction.
• Blast Zone: Mass removal on a land, similar to Ratchet Bomb but starts at '1'.
• Boompile: Fast, but random.

Answer 2

Before you build the Tron deck, I suggest understanding how the deck works.

Tron is a deck that consists of two parts: the part that helps you assemble Tron (the Tron lands, Expedition Map, Sylvan Scrying, Chromatic Sphere, etc), and the part that wins the game once you have Tron. The first part is not replaceable - if you can't assemble Tron in the first place, you'll never have the mana to cast your big cards, and you will lose. The cards you originally had in the OP - Ancient Stirrings, Expedition Map - are not replaceable for that reason. Ancient Stirrings is incredibly powerful in any case (it's one of those cards that regularly features in "which card should be banned from Modern" discussions), and should not be replaced.

The second part, however, is replaceable. It won't be nearly as effective (Karn & Ugin are very powerful after all) but it's doable. What you want to do is figure out what win conditions you can afford and/or like, and play those. I suggest using a search engine like Scryfall. You are looking for cards that satisfy these conditions:

• Cost 7+ generic mana. Not technically a requirement, but since high-casting cost cards are generally more powerful, these are the cards you look for first. You need them to cost generic (or colorless) mana because that's the type of mana that you produce.
• Win the game easily or at least have some game-warping effect once you resolve them. Remember your expensive cards can still be countered or removed (e.g. with Path to Exile). You need something that generates value when you cast it or when it enters the battlefield, or at least will win quickly if the opponent doesn't have an answer.
• Finally, it shouldn't cost too much mana. With three Tron lands out, you have 7 mana (this is a big part of why standard Tron decks run Karn: it costs 7). You can play these cards on turn 3. With four Tron lands, you have up to 10 mana. You can play these on turn 4. You probably want to avoid anything that costs more than 10 mana.

Doing the search myself, here're some possibilities. Note I do not consider the prices of these cards, because I don't know them off the top of my head.

• Wurmcoil Engine: this card gets a special mention even though it doesn't cost 7 mana, because it's absolutely devastating against any aggressive creature deck.
• Walking Ballista: this also gets a mention, because it's a great mana sink, works fine if you have lots of mana and it interacts with the opponent.
• Ugin, the Ineffable: this also gets a special mention because it is a planeswalker, which fulfills a unique role of not dying to most board wipes.
• Desolation Twin: this isn't a good card (there're plenty of decks that can ignore or chump two 10/10s), but it does quickly win the game and it has an enter-the-battlefield ability, and it's within the CMC range.
• the two Kozileks, Kozilek, the Great Distortion and Kozilek, Butcher of Truth.
• Myr Battlesphere
• Platinum Angel
• Platinum Emperion
• the two Ulamogs, Ulamog, the Ceaseless Hunger and Ulamog, the Infinite Gyre. The latter is 11 CMC, but has powerful effects.
• World Breaker

You can run whatever else you have or can afford as well, as long as they fill your need of winning the game.