I have not come across and hubs with lettering already on them from any of the usual suppliers, when I made the ones for my 3 Ruston tractor I used individual foundry letters soldered onto a brass ring and then bonded into the iron hub casting, but those letters were 1/4 high, I think the ones on the Marshall will be approx 1/8 high.
The wheel spokes on Marshall portable engines were built in two styles, I am not sure if one was earlier than the other or if they used both at the same time.
I am still thinking that 3D printed rings with the lettering on might be the answer if they can be printed with enough detail. I can easily make an STL file in Solidworks 3D.