Replacing the alternator belt on your BMW R1200RT, as well as the other bikes with boxer engines, is not complicated but it is a fussy job without the special tool. Watch here as I replace my six year old original belt and try to work around the crash bars that are in the way.
Incidentally, you could choose to simply cut off the old belt rather than wind it off as shown in the video. I wanted to try the technique before attempting to get the new one on.