No one manufacturer produces speedsters. They are not safe enough. They are not so comfortable. As for me, when I'm thinking about speedsters, it's Porsche 356 Speedster or some hot-rod. But! You can built a speedster from any roadster. This guy from Texas selected for it BMW Z4. Not worse choice!

Windshield is removed, probably here were modified safety arcs behind seats. New color, blacked headlights and tail lights, lowering and new wheels are already not necessary. They are more notable than not exciting windshield. So, if you were interesting how to do a speedster, you are in a right place!