If you are staying with wire wheels you can adapt the setup from the 75-76 CB750F. Another option is a drum-to-disk conversion kit sold by CycleX, but it isn't cheap.
In the past I have looked at the stock alloy wheels from a Honda FT500 Ascot single. It is chain drive on the left with an 18" rear disk brake wheel and a 19" front with a single disk on the left.