Dave, I think you've got the right idea but will have to get the coil resistances from a friendly service center or parts dealer. You could also just hook it up to a battery via a diode bridge without regulation and see what the charge current is.
I also wonder if you couldn't just use the 40W regulator, since it would have the capacity for either 15W or 40W. I'm not familiar enough with these magneto charging systems to help much, alas.
Best wishes,
Bruce