Finally, I managed to customize the Rocksteady action figure. Some re-sculpting was necessary here: the shoes (his trouser-legs were too long), his t-shirt (the original one was too long, so his belly was not seen) and his mouth (I really hated the stitches on his mouth).
After this, I had to create the belt with the turtle shell (cardboard paper), the band with the bullets (from a paperclip) and the narrow belt that holds the “sword”.
Almost everything must have been re-painted: the shoes (from green to brown), the T-shirt (from black to yellow) and the horns (from violet to white) and the eyes (from red on white to black on yellow) and both wristbands to be true to the cartoon show. The “sword” is the knife, that originally belongs to the Foot Soldier action figure.