You want to buy Jung (ahoocom) a beer then.
What do you need to make it compatible?
I will explain the best I can:
Numbers 1-2-4 are single keys so I can use this but will miss one:
To fit on numbers 9-10 I can use a mix of Extra number row and the normal number row so there should be
no problem here. Blanks could be possible with red colour or Grey.
I will miss numbers 3-5-6-7-8:
3 - Spacebar with different size that I think is not made by Signature Plastics.
Here is a problem.
5 - Single key that could be blank.
6 - Single key that could be blank.
7 - ISO Enter.
8 - Shift that have the size of Caps Lock key, the same used on Noppo Choc Mini and Dox keyboard.
I will miss some keys that SP can make and I can afford the price even if it's high but the spacebar is the main problem that I think can't be fixed. It's quite a mess and don't know if it's worth the effort. Do you know if over at KBDmania people have japanese keyboards? Maybe someone could be interested. It will need 6 extra keys apart from your sets but the spacebar is not possible I think.
I've never talked with SP but the size of a japanese filco spacebar is like in this pic made by Ripster:
Total lenght: 85mm
Length between stabilizers: 70mm
The central stem is at 35mm from the stabilizer stems so it is in the middle.
I will get the set even if it doesn't fit the japanese keyboard but would be cool.