My first thoughts,
Drilling the Top yoke might be easier!!
'X' marks the spot for drilling a hol, typically where I would mounts would be.
The two in the middle could probably be done away with by welding, not sure what equipment you've got to hand.
The holes at the top would correspond to the mounting hole within the actual standard clocks. I've opened them before but have forgotten what they look mount like. Again - if you didn't want to do this you could just weld the orginal clock bracket to the 'brace' I've draw that goes behindthe main headlight.
To be honest I'm not sure if its worth the hassle of having clocks!! You could get a modified bicycle computer until you decide on something more permanent. I rode my bike with no dials for a while, I knew the bike was running well so I didn't expect any warnings...