pictures please !!!
After you first posted this problem it got me thinking about just how suited the Europa is to an electronic conversion. I'm fortunate in that I'm only the 2nd owner of my car and the first guy was, well, obsessive about maintenance & record keeping. (example - I even know exactly how much petrol the guy put in the car with date & mileage ! ) So I know that he replaced the speedo head in less than 50k and the cable broke twice on him. I've also replaced the cable twice as well, so it's clearly something that can easily happen if you forget to lube it.
After seeing the conversion possibilities it seems an ideal candidate for electronics, and I wasn't even thinking about a GPS one. I think I'd be tempted if it works, especially if you could get a matching rev counter as well.
Brian
ps - I understand & respect your aim to keep the mileage log on the car, it makes a lot of sense. The original owner made detailed notes, even down to the mileage he "lost" between the cable or head unit going down and getting replaced. On the bill for the replacement speedo there's the mileage recorded on the original unit, stamped over by the dealer supplying the new one. These days we could just take a digital photo ?