Heh, yeah, this is a weird deck, limited by my card pool. I threw the Mercenaries in because of Crib Swap primarily, so I'd add two of them and get rid of the Strongarm Thug, preferably. I like the other suggestions though... Knight of Meadowgrain and Goldmeadow Stalwart are excellent.
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Sprout Swarm is an all-star if it's used right. I'd replace the Seed Sparks with Sprout Swarms, and then I'd replace either the Respite or the Spore Flower with either Thunderscape Familiar or Sunscape Familiar to reduce the cost of your green spells. Note that Sprout Swarm's buyback cost is reduced by the Familiars, so you'll really be able to power out Saprolings. Also, cut the deck down to 60 cards. Maybe ditch Fungal Bloom and a couple Sporoloth Ancients/Thallids.
Yeah, it goes into the graveyard (via dredge or one of the Spellshapers/Automaton) and comes back with either Exhume, Victimize, or Diabolic Servitude.