Just as with coins, I think the patina adds to their appeal. I would neither clean them further nor re-paint etc.
I have no idea what makes one old decoy worth $100 (about the minimum here-abouts) and another worth nearly $1M. Certainly the maker, age, rarity, subject, quality and state of preservation are important. There are decoy shows just like there are coin shows but I have never been to one. I just may attend one and maybe learn something.
If you have a way to display them, I think that is what I would do. A handmade decoy is more interesting to me than a machine made oyster can - which many of my friends collect and display.
Not saying your separated one relates...But have seen patterns using two sections of plank.
That allows building up a deke from thinner stock. An economic thing and convenience. Cured/kilned stock vs a greener block that can check and crack.
I'm going from faint memory but thinking 2" thick stock was used.
The lower section in the designs I saw used Styrofoam in the bottom piece. (The lower piece was cut into an oval basically, with it's center/inside of the oval hollowed.)
After sandwiching the upper and lower halves , a head was cut out from the same dimensional stock.