@SubscribeEvent
public void onRenderGameOverlayEvent(RenderGameOverlayEvent.Post event){
EntityPlayer player = Minecraft.getMinecraft().thePlayer;
if(event.type == RenderGameOverlayEvent.ElementType.HELMET && player.getCurrentArmor(3) != null && player.getCurrentArmor(3).getItem() == Твой шлем){
//код отрисовки радара
}