Just to update this topic, I came across this - https://www.classicautoinnovations.com/classic a day or two back. The web page is aimed at the classic car market but it could equally be used to drive a normal looking mechanical speedo on a bike.
Just in case the web link doesn't work, or you can't be bothered to click through, it's a device that gets your speed from a gps signal but then uses it to turn a cable going to your mechanical speedo. So you could have a normal looking speedometer with a cable going into it (to keep the testers happy) without needing the wheel driven gear mechanism. As the cable comes out of a black box with some wires going to it who knows what it's measuring.
As usual, as it comes from the US, the brexit pound makes it look a bit pricey but hope springs eternal (as they say) that it won't last forever
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