-MIKE- writes:
https://photos.app.goo.gl/DqXrs5DI2roJTli22

Remember, the existing 2x12 will not come out in one piece or even
several pieces which could be reassembled to use to trace the "arch"
onto the new board. It's pretty much, mulch, held together with paint.

Any ideas?

Take a photo, enlarge it, then trace the arch.