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My minimalistic take:
Klutz is useless on him. Rather than a new ability, simply giving him Trick or Switcheroo (or a new Ghost equivalent) would alleviate this. If not, replace Klutz with Battle Armor.
Next, he does need stronger Ghost STAB. Shadow Bone, Shadow Force, Spirit Shackle, and Spectral Thief are all exclusives right now, and none really fit super well.
If he can't get a new move, buff Shadow Punch to 70 damage rather than 60.
There, he's a bit better and just as awesome as before. No stat boosts or OP abilities needed.