Mine are off a 2003 YZ450. 46mm lowers. The bottom tripple is around 30mm wider than the standard 650 tripple, so may have clearance issues on the 650. ( mine on GSPD)
The other option for the front end is Honda XR400R forks. Not sure on the size, but I know the XR250R is the same size as the BMW, so dont need to change tripples.
These are cartridge forks, so are modifiable.
Just another option that maybe cheaper again.
Check out FAQ at www.f650.com
section on fork upgrades.