Rummaging through one of my electronic boxes, I found these M3 'standoffs' for mounting PCBs. There was an M3 x 0.6 die in my collection of oddball tools so I ran it down the thread of two of these. The result is not beautiful but there is enough thread to hold a plastic tail light in place.

I now had an adapter that I could screw into the JIS holes in the tail light. I bought some shorter screws from David Silver and they turned out to be ISO metric and stainless. I was able to lop around 10mm off these so that I could screw them into the adapters.

With the gasket in place the rear light lens fits nicely and I feel happy in that I haven't had to butcher any priceless parts.... :-)

G