Won’t cost a thing to come up with the design . Just brain power then you sell the design to a manufacturer. Trust me I’ve seen this done on forums before . And the end products sold well not a bike but a flashlight
I've got a good plan on a saw engine based on a yd100, ms440, and a 38mm stroke. But it isn't anything new, the engine is basically a phantom, just with some personal touches.
I think a steel tank frame, 4 stroke engine, and a charging system. Electric start would sell too. This is kinda what I want to build as my own version of the phatmoto.