"Somebody" was telling you lies mate, all the K1-K6 and F/F1 carbs are 28mm and apart from some slight changes, were pretty much the same. The F2/F3 carb (and K7/K8) is a different style of carb with accellerator pumps, and work real well on the other engines too.
I had a set on a K2 engine in my K1 "Kruiser" a few years ago, and I could tell the difference right away, better accelleration and no "flat spots" that the old K and F carbs are famous for.
If you want to use F2/F3/K7/K8 carbs on your bike you'll need to buy the carbs, choke cable and knob, air box (larger inlet holes than yours) throttle cables, and K7/K8 inlet rubbers to adapt the carbs to your head. A bit of stuff to buy, but still much cheaper than CR race carbs, and easier to set up. Cheers, Terry.