Now I'm making real process! I fixed the model and added the vest. Now I just have to fix the color of the hands and remove that weird thing at the shoulder of the jacket, and it's good to go.
Looking surprisingly decent enough for a simple PS2 retexture! Now I'm just trying to figure out how to change the color of Jimmy’s arms to their normal color, because it changes whenever I change the color of the school vest, that's why they look so pale here. (Overall I think I'm near of making the texture look properly fixed).
I just found out that you can create your own custom models for the AetherSX2 emulator, so I made a little test and added gloves and upscaled the original leather jacket! I hope to actually make it look like Johnny's jacket in the future. But I think I'm the first one to ever mod the AetherSX2 version models of the in-game files.
Edit: Oh by the way, the model with the gloves is showcased here if interested: