Mechanical speedometers have no voltage specs, it's the bulbs. 12v bulbs are easy to source, easier than 6v and those aren't exactly scarce. The Lifan version of the K0/HK0 speedometer will fit your K0 style bucket bucket, although you will have to rig something to take up the slack of the retaining clip. Otherwise, it won't be snugly seated. Ebay seller "webllc" usually has the 70mph version for around $25, complete with 12v bulbs. It's pretty much all-plastic, with a black outer trim ring. Your only other option it to find a vintage Honda unit, from another model bike, there are versions calibrated to 60mph & 80mph; these fit the headlight bucket perfectly, but aren't common. Used versions vary greatly in both price & condition. Plan on spending more like a buck-and-a-half for an NOS example, if you happen to find one.
As for sourcing a longer cable, the only viable possibility is to source one from another model Honda. I believe that K1 & later CT70 cables are ~1.5" longer than K0. CT90/110 cables might be longer still. There may be other, larger, models with even longer cables that have the proper fittings at both ends. The problem is that no one specifies cable length, so it's an "experiment of one"...you.
HTH