Create your own. Start thinking like a programmer. You need to create your own ItemStackTileEntityRenderer which handles your Spear item. God why do you have to have everything put right in front of you on plain paper.
Step 1. Create Your own ItemStackTileEntityRenderer implementation and override renderItem(ItemStack)
Step 2. Duplicate what the Trident does, but with your own model.
Step 3. Set it in your Items Properties with the Item.Properties#setTEISR
Step 4. Create thrown entity by extending the TridentEntity
Step 5. Create renderer that renders the your spear model at the entities position/angle etc.
Step 6. Make sure everything works.