Another basket case project, but with potentially a different end goal!

Yeah, I explored that too. I’ll have another look later today.

Just to really add to the confusion, the early automobiles when they started converting to electronic instrumentation, encoded the speedometer off of the driveshaft. I would think a single magnet on the drive shaft up near the transmission would suffice since the driveshaft is spinning around 3:1 ratio to the rear wheel.

Just something to think about.
KB

The issue will be where to mount the sensor, but I’ll have a look at that too.

I still favour the front wheel apart from the cable issue.

I put a magnet in one of the holes in the rear disc & hid the cable for the sensor on the underside of the swing arm. You can extend the cable, it will still read OK. I put the sensor on the forward edge of the brake caliper, so again, you can't see it. I had the same model of Acewell as you before, they're good units, I only changed it to a model with a needle tacho as I found it difficult to read the digital segments on the tacho sometimes. For setting the speedo up I got it in the ballpark, then attached a sat nav to the top of the tank to get an accurate speed then fine tuned it that way. I set it so that it over-reads by about 3 mph, as most speedos do.

Ok, decision made; rear wheel, magnets on the disc bolts and the sensor attached to the licence plate light bolt. When the licence plate goes on, it will hide the whole thing. Pulled a new black wire to the front and you can't see a thing. Happy!

As the process for finding someone for the exhaust build has stalled, I cleaned the pipes that came with the bike that I initially had written off as rusted and broken. They came up surprisingly well and there are only a few small holes that I can easily weld shut. I think I'll hang those on once I fitted the final missing pipe that I will getting tomorrow and at least I have something so I can trial start the bike. The system has collector boxes, but they are rusted out inside, so it will still be loud!

I don't know where you're based in the UK McBoney but Lee at Steelheart Engineering in Kent part fabricated the exhaust on mine in stainless some years ago. PM me if you'd like his number.