Here's what Frank Ford does,
http://www.frets.com/FRETSPages/Luthier/Technique/Guitar/NeckReset/BCRIchReset/bcrichreset1.html. I suppose you could use your Japanese saw since the fretboard has already been cut. Does it have a truss rod? That could ruin your saw if you are not careful. Another option is to remove the fretboard and put in a shim to correct the angle. Or you could taper a new fretboard that would correct the angle. That would solve the problem of the old repair where the fretboard was cut off and maintain the intergrity of the Spanish heal joint.