If you have the picture of it scanned into the computer, i would suggest that you open the image up in Inkscape, or your choice of vector programs, and manually create the outline. As much as I love technology and I do periodically use the tracing functions, I would probably not for the stencil.
I have made a few of these for my wife. That is the process I follow. Picture, with ruler so I can get scale. Manually trace. Cut out of acrylic.
BTW, the Glowforge also makes for a really convenient tool when looking to make a specific triangle ruler for cutting out pieces.