Just opened my 2002 1150 at 62k miles for transmission input shaft inspection/lube. Splines perfect fortunately. Looking at the shaft (instead of a picture) for the first time I saw one of the reasons for failure. The shaft is TINY. Read here about an improved clutch disk with a longer hub which is certainly an improvement if the shaft hasn't trashed yet.
But if the shaft is stripped or on it's way I think I've seen $1G for a new shaft plus a transmission tear-down.
Since someone has the capacity to install a longer hub in a clutch disk, then they could just as easily install a BIGGER hub. Larger OD. (Stay with me) Now add a simple machined collar over the damaged shaft, old spline size for the ID, new, bigger splines for the OD. The stripped ones appear to have enough spline left to hold the repair collar, just in the wrong place to engage with the clutch. A snug fit and Stud and bearing Loc-Tite or Caterpiller branded epoxy would easily hold it on. The only force applied longitudinal is generated by the riders lift hand on the clutch lever. Cheap, easy, straight-forward manufacturing, no transmission tear-down.