So, here is the finished article. The ammeter has been stripped & cleaned inside & out and is now working happily.
The chrome on the rim is good, and the case has been repaired.
This is the new ignition light. The design & material selection is inspired from the genuine ones I have seen on eBay and the various forums, particularly the C10/C11/C12 forum. It's not an exact replica of anything but it looks the part.
I've made a carrier from 2 mm Tufnol which is a little thick, but all I had. The lamp bracket is made from 0.5 mm brass sheet, with insulation gaskets as necessary, riveted together. The one thing I would like to find now are a couple of 2 BA thumb nuts to replace those steel hex nuts.