Fixed your picture
I would use some cardboard to make a pattern and cut it out with a jig saw.
I saw your post on CNC Zone, and a tabletop CNC router might work if it is big enough. But trying to model the arcs might be a bit tricky. What I might do in that case would be to take a good picture as a top view, with some known points marked so you can scale the drawing in your CAD program. Then you can fit the arcs to the picture. I have used this technique for reverse engineering shapes that are difficult to measure.
Wrap the risers with kerf-sawn solid or plywood. The treads are usually 1" thick oak.
How about the nose? Should I get one of those lj Smith bendable noses? Or are they no good?
Woodman's idea of kerf cutting will work great . Saw cut almost completely thru the riser vertically every inch or so and bend it around the curve. Or you can buy a product called bendy ply or wiggle board. bend it to your pattern and glue two pieces of this together and clamp in position. then cover in hardwood veneer. Or
Use 1/4" hardwood veneer and it may bend with ease.
Cheers
Martin