The market for JIS headsets is simply too small for me to bother doing a special run of parts for an already niche product, especially when there are options to rework existing parts to satisfy any need.

You could, as noted, adjust your frame and fork to ISO specs as one option. Another, even lower effort option is to just press the ISO spec cups into the JIS frame, something which apparently gets done quite frequently without issue. The crown race ID is easily opened up to JIS specs with an adjustable reamer (I can do this for you). If the thought of using JIS cups in an ISO frame is too much for you to bear, we can talk about modifying those, too.