All the Italian bikes look alike to me.
1957 Perugina 175.
http://www.wheelsofitaly.com/wiki/images/thumb/a/ac/1957_Perugina175cc.jpg/350px-1957_Perugina175cc.jpghttp://www.ffac.it/RD%20Series%20Club/Eventi/FMI%20-%20Magione%202010%20bis/motoperugina175cc3.jpg
A 1962 Greeves 25 DCX.
The aluminum casting used in the frame finally clued me.
And I couldn't find your original pic, either.
http://image.invaluable.com/housePhotos/chorley/07/319507/H2603-L30143846.jpg
http://galleryplus.ebayimg.com/ws/web/321257553067_1_0_1/1000x1000.jpg
http://4.bp.blogspot.com/-u8iEeCV_rWE/UvuPGYxShHI/AAAAAAAALQs/eKNQk4e8JC0/s1600/800px-Greeves_250DCX_Sportsman_1962_.JPG
It is a 1965 Moto Guzzi Stornello 125 Sport.
http://www.msc-sernatingen.de/haldenhof/images/2007teilnehmer/HOFFMANN%20Manfred,%20Salem,%20Moto%20Guzzi,%20Stornello%20Sport,%201965.jpg
According to this 2005 article, there is currently just one production diesel powered motorcycle; it starts life as a Kawasaki KLR, and is intended for the NATO military:
http://www.gizmag.com/go/4272/