Aren't they mostly extruded? I think the down tube is sometimes forged. I don't know much about aluminum manufacturing, but it seems like making a prototype from welding sheet aluminum would be a good way to start. Once the initial design is complete, you need a manufacturing company to build it. Then you would need to go through some redesign iterations to come up with a good combination of durability, cost, and weight.
Manufacturing from aluminum is more complex than from steel. I remember in the mid 1970's how riders dreamed of aluminum frames for motocross bikes instead of chrome moly. It finally happened, though it took 20 years of R&D and manufacturing advances to make it possible. As a wild guess I would say you would need to spend $50-100 thousand in prototype spins before you end up with something competitive. A cheaper alternative might be to get a Chinese manufacturer to customize one of there existing designs.
If I were going to try manufacturing something for scooters I would start with a simpler part like handle bars. I think breaking even on scooter parts manufacturing would not be too easy. Not only do you need a competitive product, you need successful marketing and advertising. The field is crowded. To sell a scooter part means taking business away from an established company.