I'm on SBN (the sportbike network)
My bike is still strong as ever after 30k of hard riding; this includes the clutch with has happily withstood many passes at the drag strip and even more clutch-up wheelies.
There is a '93 ducati monster 900 on the forums with over 158k on it (still riding around too), and ducatis are well known for being a PITA to maintain. There is 80k on a ZR7, an old honda with 288k, an F4i with 144k+, several other bikes with over 100k.
Generally, sportbike engines will last a good while as long as they're maintained. Put in some shell rotella synthetic all their lives and they'll stay fresh. Give her the old italian tune-up every once in a while and don't lug the engine around and it'll be happy.
The reason you don't see many with high miles is they usually get wadded up before they break down