Cant comment on this in too much detail as I dont know what your orginal brake caliper is. but on KTM super motos when you swap to the 21" front you have to replace the 4 pot caliper with a 2 pot sliding one as the clearance is less with the 21" wheel, so if you have a 4 pot opposed piston caliper my guess is you need to get a slimmer 2 pot sliding version. Your master cylinder will work but may be too fierce if its for a 4 pot brake also it makes the swap easier to have a caliper /master cylinder all cabled up so you dont have as much bleeding to do if swapping back and forth (I know as I did this swap on a KTM Duke).