Maybe this can help with your confidence in the clutch. - I installed an engine from a 1989 K100LT in my 1988 K100rt in June 2008. When I purchased my RT the guy assured me a new clutch had recently been fitted. At the time of purchase the RT had 128,000 km's on the clock. I did quite a bit of touring in Victoria 2-up and fully loaded in the first 18 months of owning the bike but the output shaft was noisy (loose rivets - ex police bike). When I exchanged the engine at 147,000 km's I examined the clutch plate and the lining was measured at 5.4mm thickness (suspect 5.5mm new), and the BMW part number (in whte marking) was still visible on 1 side of the lining face, I kid you not !
To put it into perspective, the clutch plate is pretty much identical to that in an early VW beetle, and with K bikes being relatively low geared in first gear, the clutch is likely to last a very long time, unless you feel the need to thrash it by slipping the clutch and taking off in second gear to make the K reach the stated 100 K in 3.9 seconds, which by the way is both achievable and still respectable, even by today's standards. Long live the K bike I reckon !
Hope this helps.