Well after turning the shed inside out for a couple of days looking for the bridge piece three turn up at once.
I sprayed the best two up in white and did the airscoop too.
A night or two later I went to fit a battery in the battery tray and at this point discovered that there was no way the SIP 9Ah battery I had intended using was going into the space and also that there was little chance of getting any sort of battery in at all from the side. Out comes the tool box and the jigsaw was applied to the side of the Vespa tray.
A standard YB5 type battery fits nicely.
I've temporarily connected it to the main loom and still have lights. I have a load of spare wirey bits like the SIP control box, battery isolator switch and the fuse holder to try and recall how to wire up now.
Meanwhile having discovered the horncast I'd spent hours prepping and painting in English blue doesn't fit I start comparing it with my original one. I found that it seems to be about 3-4 mm narrower than the original Italian one. The top and bottom edges actually fit inside and it catches on the legshield bracket. The hole centres are also about the same 3-4 mm closer together.
So I dig out my old one and start to look at prepping it for some RAL5020 Ocean Blue which I'd ordered as a test to see if it was any closer to what I think the GP blue should look like. As I prep it I find a few traces of the original blue under the flaking paint.
Once painted this looks like a much more promising match than the the English Blue of the other one.

"Our dilemma is that we hate change and love it at the same time; what we really want is for things to remain the same but get better."