I'm using the std CB750 master cylinder on my K2 with twin discs and have no issues with the travel or feel. I think you will find that it is not necessary to change the std M/C to one with a larger piston, such as the one used on the GL1000's.
However, if you feel happier with that then go ahead and fit one with a larger piston. As to using a 'modern' one (I assume newer) there will be no advantage over and earlier one that has the same piston size, though clearly a newer one may have better/newer seals but as long as the seals, and bore are in good order they should both perform the same.