The best card for replacement is Vectus. It's not perfect by any means.
With Kanrethad Prime + potential Youthful Brewmaster combo it is a 10 mana turn; summon boarfull of minions and make favorable trades with your 2x Nightshade Matron's that you dont fall behind in tempo.
With Vectus + Youthful Brewmaster u'll have 7 mana turn; summon small minions that don't do anything immediately. At maxium you can have the leftover 3 mana used in a form of removal with Blood Shard Bristleback.
Also when using Vectus, U'll have to remember that there's actually 3 deathrattle minions in the deck; Taelan Fordring, Death's Head Cultist and Envoy Rustwix! Just keep it on your mind when playing the game and play the cards in order that gives you the value or healing you'll need in that specific situation.
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Just played against this list on ladder and immediately searched it up. I love seeing lists that hone in on one mechanic (primes) like that. Ultimately a real nail biter with a few bad decisions on my opponents side and I managed to narrowly beat this with spell mage. This was in legend by the way. Not particularly high or anything but the deck managed to utilize every single aspect of how you built it. Just wanted to let you know cause it's the kind of thing that I'd be happy to hear if I made a deck list this interesting. Keep it up, you're really on to something here and the talent to back it up!