Ducati Single Kickstart Spring 200 250 350 'Heavy-Duty'

To suit any narrowcase bevelhead single, to my knowledge.  Ducati's original part number is 0603-07-010, the last version of the part before the whole kickstart mechanism was revised with the widecase models.

The kickstarter has always been the Achilles heel of the NC engines.  While not much can be done regarding the troublesome gears, this spring is wound with slightly thicker metal and about 15% more tension.  If your kickstart lever doesn't really return like it should (very common), this should help.