Hi Mick, there's no substitute for cubes.
All the valve train is common across the heads so the towers, cams, rockers and springs/retainers can all go on the N42.
Not sure about the cam grind being suitable sorry.
Don't bother with bigger valves, the N42 are big enough.
Definately flat top pistons, you want the compression up over 10:1
Good luck with the build.
Roger