I made the same mistake years ago, then bought calipers to always measure and be certain. The cup diameters are so close (30.0 JIS / 30.2 ISO) that I don't think it's an issue to mix and match. If your original crown race isn't pitted, I'd say it's worth a build up to see how it fits/works.
What brand/model headset did you buy? And what was on there before?