I have been looking for a Smiths tacho for my Silk, the 80mm RVC type seem to be getting silly money on Ebay at present (£80+ for an untested tacho?). I took a £12 punt on a french Jaeger 80mm electronique tacho, 12v, 4 cylinder 4 stroke (not an english Jaeger made by Smiths) and it arrived today. Looks well made with only two wires to connect instead of the usual three (found a Jaeger manual in french on the web to work out the connections). Tried it on the Lumention ignition system and it works a treat. Looks like a low cost option if you are not bothered about originality i.e Veglia.
After looking for a Veglia for quite a few years without success, I got the Jaeger. A week later a Veglia came up and I got that too! Works a treat. That seems to be the way it goes.