I want to hit my (low-level) players with an animated ship's figurehead. I was toying with stats for the Sea Hag, but an animated suit of armor might work better. I'm thinking the damage resistances would be less, but it would have similar condition immunities.
Animated Armor doesn't have any damage resistance, but it does have Immunity to Poison and Psychic damage. I think you should keep those. I'd add Vulnerability to Fire damage - it's wood, after all.
I'd also lower the AC - wood would be less protective than plate armor. You then have a choice of increasing HP to maintain the challenge, or making up for that in some other way - more opponents, environmental hazards, more damaging attacks, etc.
Since you're dealing with a low-level party, increasing the damage of attacks could be lethal. You might be best off adding another attack option. My suggestion:
Animated Figurehead - as Animated Armor (MM 19), except:
Entangle. Ranged Attack: Range 30', one target must succeed on a DC 13 DEX save or be Restrained until they use an action to make a successful DC 13 DEX save.
It depends on what your end goal is.
Do you actually want the physical change in material to be represented by stats? I.E. wood might be less protective than metal so wood should have a lower AC. Wood might be more durable than metal so a wooden statue should have more HP.
Is this change even going to be noticed? Have your players really memorised the Monster Manual so well that you're forced to throw them a curve ball by changing a monster's stats?
Are you actually trying to lower the monster's challenge rating to represent the change in material?
Some other reason that makes this change absolutely imperative.
If the answer to any of these questions is yes than let me direct your attention to page 246 of the DMG. On this page is a list of ACs for an object based on its substance. Wood is given an AC of 15, close to what Kevin suggested, and iron or steel an AC of 19, close to the Animated Armour's. Unfortunately the hit points given for object size aren't going to be much help in this case but that only matters if you're trying to change the creature's CR.
You can then adjust this AC up or down as you see fit, along with its Hit Points, attack bonus, and/or Damage per Round until you get the desired Challenge Rating as per the monster creation rules starting on page 273 of the DMG. You can even give it vulnerability to fire if you like.
If your end goal is merely theatrical than go with occam's razor; change your description, not the stats.
Changing a creature so drastically can be quite frustrating, time consuming, and at the end of the day may not even matter. There's not much point in going through so much trouble if it's not needed. The DMG's first suggest on monster creation is to find a creature that matches your idea as closely as possible (which you've already done) and make minor changes. One example they give is changing a Giant Eagle or Roc into a Phoenix, just slap on some fire immunity, allow it to deal fire damage and voilà. Same thing can be done with Animated Armour, just slap on some fire vulnerability, describe it as being made of wood, and call it a day.