that is not a stock triple clamp.
it may have been at one point, but whomever you got it from modified the heck out of it. The bar mounts look to have been filled with aluminum rod (poorly I might add since you can see some lingering porosity) and shaved off the gauge mounts.
Without drilling or welding you aren't mounting anything to that top clamp, and honestly if it an original cast piece that has been welded on, I wouldn't drill into it for fear of cracking it.
I have in the past used handlebar clamps to mount a mini speedo, but if you have that clamp you are running clipons and you probably won't have the room. My best advise is to get a digital bicycle speedometer and mount it to the back of the headlight with some strong velcro or 3m tape.
this is assuming you are not good at metal fabrication. If you are good at it, why not make a speedo bracket that replaces the washer under the stem nut? Shouldn't be too difficult a bracket to make if you have access to a metal band saw and a bending brake.
