When I got my X1 I wanted rid of the original back end (and hence the numberplate light). Having sorted the looks of the back end out I got a rear lens with a clear area (to light the number plate) from somewhere in the US (American Torque?) when I ordered some other stuff.
I figured I should do something similar with the S1 but the cost is silly for what it is.
Anyway after a little searching the lens is the same as a Ducati Monster (easy to find that one) but also a Derbi Senda 50 scooter (Note some of the monster lenses have no clear portion).
I found a scooter spares place called 'racing planet' sells a vicma branded lens (with a clear portion and) for the princely sum of £3.67 (+£4 P+P).
http://www.racing-planet.co.uk/light-derbi-malaguti-gilera-p-137615-1.html?backcPathco=21&cPath=738_19136#.VL6XdyusXkoOrdered at lunch yesterday, arrived and fitted today.
Just thought this might be useful to someone else.