I've used the f650gs single front wheel (it's a 19") and had woody build me a rear from a 800gs hub and Exel Takasago rim. I stayed with the same size as I'm vertically challenged but check out "A"'s thread. He started with the 650 twin and put a KTM front end on and i'm sure it's an 18" rear. Not 100% sure if the Dakar front will work but take a look at the schematics on BikeBandit and see if the 19" GS and 21" dakar have similar hubs and brake rotors.
I'd be a little concerned about putting a 21 on the front with the existing forks. there might be clearance issues with full fork compression.
I've done a few more mods since this photo... 2" rox risers. painted lower panels charcoal. removed front fender and added upper rear 650 single front fender. Tail tidy is next.