I had been just dealing with the lack of speedometer, and I wasn't actively looking for another solution. Fortunately here in the USA if we register as an antique there is no safety inspection or MOT. Sometimes targeted advertising is actually a good thing. In my news feed of a popular social media up popped an advertisement for a GPS Speedometer. Brilliant! Why didn't I think of that! It needs no cable, just power and a clear view of the sky. The antenna could be hidden anywhere on the bike or just strapped on with tyraps in plain view. It would look plain and not unusual unless you really looked hard at it to find the digital display.
Nobody makes a GPS speedometer for an 80mm diameter speedometer bucket. They are all 65mm or 85mm. My grandma used to say "Can't is a coward", meaning if you think you can't then you are already defeated. There must be a way to fit a 85mm speedometer in a hole for an 80mm unit. I bought the speedometer for the modest asking price and resolved to figure it out.
ELING MPH GPS Speedometer Odometer 140MPH For Auto Motorycle With Backlight 85mm 12V/24V
Upon arrival like any good engineer, I took it apart to see how it worked. The mechanism was simple. The bezel pried off easily exposing the bucket with the speedo unit inside. Two screws in the back removed, and the speedo pushed out of the bucket releasing the power connector inside. Another connector went to the GPS receiver hot-glued to the bottom of the bucket.
The plan was to cut the speedo bucket so that half was above the headlamp and half was inside. I would then contrive a way to join them back up.
Luckily there is just enough room around the edge for some stainless steel sheetmetal. The sheetmetal was fastened carefully to the inside of the bezel and passed along the inside of the speedo bucket. It could then be re-attached by drilling a hole through both and allowing the sheet metal to self-tap.
All assembled, but there was naturally another challenge to overcome. My bike is 6 volt positive ground and the speedo needs at least 9 volts. No problem. I bought a boost regulator and set it for about 12 volts. It gets powered from the original speedo backlight and there you have it.
MT3608 DC-DC Boost Power Converter
One night's worth of work and I have an operational speedometer. I can take my time getting the original rebuilt and nothing on the bike was done that couldn't be reversed. A big fat o-ring from a spin-on oil filter covers the bottom of the bezel which had to be thick enough to support putting four screws in it. The o-ring could be backed with some RTV if I thought the bike was ever going to get wet... which it usually doesn't.
The maximum speed is a bit optimistic.