Your riding style, the terrain in which you are riding, and other factors will influence the best gearing setup, however BMW has found that for tight, enduro-type riding 13/48 works well (and may require removing one link from the standard 108 link chain) while 14/50 works well for moto. We've found 14/48 (one down on the countershaft and stock rear) to work well for all around riding.
Special order item from Germany takes approximately 3-4 weeks.
BMW G450X Rear Sprocket
BMW Original Parts