I posted my request on Scooter BBS and thank you to John Stafford, who suggested to use a web time machine to look up the URL of scooter culture and low an behold I now have my list of metric sizes for the hardware and fasteners.
Your best bet is to open the link and create a PDF and print it. If that does not work click on the JPEGs below and print. I can not post PDF documents or I would and it would print better.
The fender to steering column bolts are 5mm x .80 x 18mm hex head bolts, no markings on the head. Good place for stainless since you can polish off the bolt head markings, and it's a non high stress location.
Furthermore, in speaking with Collin W. and Dave C. said:
"On a more serious note on nuts and things....
Vespas are pretty straight forward in this respect. Perhaps, this is why no list exists - it isn't strictly needed? There are a few odd-ball threads:
The earth stud is, for some unaccountable reason, M3.5, so needs a wingut and plain nut to match (Tap out an M3 when stuck).
The front suspension damper top nut is M8 with a fine thread pitch of 1.5 (I think?). This was, originally, a 14mm wrench drive size. Try getting hold of one of them!
Early(ish) GS 160's had M6 brass wingnuts holding the rear light in place (Bizarre!).
For the same GS's and earlier models, the gear adjusters were, also, brass and had matching lock nuts.
The M7 plain hex crankcase nuts can be difficult to source in the UK away from a scooter shop.
The seat/tank retaining bolts are also M7 (Later Vespa models did the sensible thing and were fitted with M8 bolts).
Headlamp rims are held in place by M4 raised countersunk slotted bolts (15mm length).
God only knows what type of threads are used on the carbs (Actually, God AND Delorto!).
Think that's about it on the dodgy sizes and types?
Apart from those items listed above, I can walk into my local fixing suppliers and buy everything else across the counter - even if I ask for STAINLESS STEEL?" -- Dave C.
If you know of another list or have one that is more complete or has different stuff on it please let me know. I would like to offer a Vespa hardware list as complete as the Lambretta hardware list that Gene, from Scooters O, published.