So, there's a ridiculous frame manufacturer out there that has chosen to put out JIS forks. I recently had the misfortune of buying one of these clown's framesets.

There aren't a lot of options in the JIS headset game, so I'm wondering if one can use a JIS race along with a standard ISO threaded headset.

Thanks for the help.

Ok, is the fork set up for a JIS race (27.0mm) or ISO race (26.4mm)? If it's a steel fork, it's trivial to mill a JIS race seat down to ISO size; then you can use any ISO headset. It's harder going the other way (mounting a JIS race on an ISO fork race seat). You either have to build up the race seat with brass and re-mill it to JIS, or use a knurling tool to expand the ISO race seat to hold the JIS race. The first method is preferable, if you have a choice.

Otherwise, some headsets were made in both JIS and ISO dimensions; buy one of each and mix 'n' match to make it work.

just to reaffirm JDT: you can just mill down the 27.0 fork crown to 26.4. there's a tool that does exactly this, and a good shop should have no problems doing this.

So what would happen if I simply put a JIS race on, then used ISO for the rest of the headset parts?

If it's the same model headset it should work fine. Mixing manufacturers and models complicates things, though. And make sure your frame isn't also reamed for JIS (30mm) cups. If it is, you'll have a hard time installing ISO (30.2mm) cups. If that is the case, you may as well get both the fork and frame re-machined to ISO specs and be done with it.