Okay folks, The odometer reset shaft is 5mm at the head with a detent for the set screw. The hole on the reset knob is 5mm as well, and the set screw spacing aligns with said detent.
The radio knob fits over the reset shaft but is too loose. Even with a set screw, fitment would be sloppy.
The headlight knob threads onto an M5 shaft. That means that the headlight knob has a diameter of about 4mm. The knob size is a very close match both in height and inner diameter. If you put the knob in a vice and use an M5 drill bit to the aluminum threads, you can quickly make the HL knob a perfect fit. Because the sleeve is aluminum. You use a real set screw from your favorite fastener shop. Drill a matching size and tap the sleep. It reads like a lot of effort but the total duration could be measured in minutes. It will take longer to source the tools (and possibly cost more than a genuine reset knob). However, if someone like
@HARRY Malkasian was so inclined, he could scoop up HL switches and convert them to reset knobs.
The set screw itself is tiny, and an odd size.