I'm not familiar with the Veglia counterweights, but am with the Motometers. They changed over the years - in the early 70s they were small brass thick washers. Then they were molded into the hub. And finally in 78 the needle hubs were plastic with a light weight needle made from beer can thickness aluminum. No counterweight. I've never experienced the phenomenon you mention of the needle going to max and staying there because the counterweight was missing. Seems odd to me. I'd think, without the counterweight, the needle would jitter and shake a lot, not read max and stay there.
I think your easiest solution is like Roy says - solder. Perhaps find a washer with an inner diameter about what you need for the outside D. Use it for a form to melt some solder.
If I remember, I'll take a look today at one of my spare Veglias and get back to you.
