Are wheel spacers safe on a JDM car?

Yes, when they're hub-centric, correctly torqued, and paired with enough stud engagement. A quality hub-centric spacer carries load through the hub just like the wheel does, so it isn't relying on the studs to center. The rules that keep them safe: match the bore and bolt pattern, use longer or bolt-on studs past roughly 15mm so full thread grabs the nut, and re-torque after the first short drive.