I fitted an rvf 400 front end to my 21 and it was pretty straight forward.
You need the forks (surprise), yolks, bars, mudguard, wheel spindle, wheel spacers, brake calipers and discs. swap the stem from your original yolks and press it into the rvf yolks. this involves grinding or machining the weld which holds the stem to the yolk, pressing the stem out of of the yolk and into the new one and welding it in.
On my 21 I had to modify the lock stops as well because the r/h fork fouled the rad cap. I guess on the 28 you would need to cut the ignition switch mount off of the top yoke as well.