I actually think that Ricky's suggestion will work well. I looked into them last night and found that, though not cheap, the Piko wheels do look the part and are cheaper and lighter than cast-iron pieces. I am also going to look into 3D printed parts. I could always design something if I need to. I may even have a friend who has a 3D printer...If you are struggling with driving wheel size, why don't you build GWR 68xx Grange class instead? - near identical boiler, superstructure and wheelbase - just smaller wheels!
And, to be honest, I am going with the Hall class because I want to build the Hogwarts Express (with a few minor changes).