I said this in other comments earlier, you don't need to rewrite Unity to build your game. Build what you need, or pick up an open source product and add what you need. I don't understand why people bring up financial feasibility if you're being charged now for a wrong choice in the past. This was to be expected. It's always the same pattern. If you can't figure out how create your game without some false promise product, then don't build your game. It's really as easy as that.