Now for replacements, we're going to stick with cards that are as close as possible in mana cost but are still good in a Meren reanimator strategy:

Activated Sleeper can be replaced with Vile Entomber.

Walking Ballista can be replaced with pretty much anything. Some options I would consider are Weatherlight Compleated, Gray Merchant of Asphodel, Underrealm Lich, Creakwood Liege or Old Stickfingers.

Budget Options
Lands

Blooming Marsh, Darkbore Pathway, Deathcap Glade, Nurturing Peatland, Overgrown Tomb, Undergrowth Stadium and Verdant Catacombs can all be removed to save a good chunk of money and replaced with Darkmoss Bridge, Temple of Malady, Jungle Hollow, Woodland Chasm, Witherbloom Campus, Foul Orchard and Hissing Quagmire.

Phyrexian Tower can be replaced with a basic Swamp.

Prismatic Vista can become a basic Swamp or Forest.

Urborg, Tomb of Yawgmoth and Yavimaya, Cradle of Growth can also become a basic Swamp and Forest.

Sorceries

Reanimate can be replaced with Persist, Victimize or Footsteps of the Goryo.

Agadeem's Awakening can just be replaced with a basic Swamp.

Toxic Deluge can be replaced with Culling Ritual.

Natural Order can be replaced with Pattern of Rebirth.

Artifacts

Ashnod's Altar can be replaced with Bloodflow Connoisseur.

Phyrexian Altar can be replaced with Bloodthrone Vampire.

Birthing Pod can be replaced with Vampiric Rites.

Enchantments

Journey to Eternity can be replaced with Awakening Zone.
Creatures

Birds of Paradise can be replaced with Dockside Chef.

Dauthi Voidwalker can be replaced with Weatherlight Compleated.

Yawgmoth, Thran Physician can be replaced with Shadowheart, Dark Justiciar, Midnight Reaper or Skyfisher Spider.

Wurmcoil Engine can be replaced with Noxious Gearhulk or Massacre Wurm.

Protean Hulk should probably be replaced with Sheoldred, Whispering One or Virtue of Persistence.

Natural Order and Chord of Calling can be replaced with any utility cards of your choice. Some I like are Victimize, Perpetual Timepiece and Apprentice Necromancer. Or if you want more removal, Dictate of Erebos, Lokhust Heavy Destroyer and Nekrataal are good options.