How do you make modules if you have a sample that needs to be ported to the game?

Recently noticed a mod on megamix+ with sonic, there was a desire to make a similar mod with your favorite characters from another universe. There was a question of how to port models. I do not know how to create new ones, but there are original ones, which do not fit the format (as it seems to me, in the extend parts of the body, the models and textures are located separately, and I have 1 model, covered in layers). What could be done about it?