So I thougt about it and had the idea that I maybe can replace the itemRenderer field in the Minecraft class using reflections. I created a new Class which extends ItemRenderer and replaced the field in MC successfully. But the problem is now that my changes aren't applied (renderItemInFirstPerson-Method which I've overrided) and I don't know why, because when I check whether the Minecraft.getMinecraft.getItemRenderer() is an instance of my own, new ItemRenderer it returns true Do you have an idea why?
Edit: I now got it, so you have to change the itemRenderer in EntityRenderer, too.