As far as I know, only the IBM buckling spring keyboards and perhaps a few KeyTronic keyboards could be modded to a different layout. They use the same membrane/PCB for all layouts and have two plungers underneath those keys that are wider in some layouts than others.
However, most keyboards have the switch in the centers of all wide key, and so do the Realforce keyboards, as far as I have seen. I think also that the Realforce PCBs are unique for each layout (ISO/ANSI/JIS).
Besides the Kana and Del keys, the Left Shift key is one unit bigger and the Right Shift is one unit smaller in the JIS layout compared to ISO layout.