Hmmm, could you maybe replace the contacts of a little-used button ("insert" or "scroll lock" or something) with a minature push button from RS or Maplin, and stick the original button to the top of the new switch?
Or, you could find a panel-mount pushbutton with a shallow back end and mount it in a space on the keyboard...
I know neither answer is ideal, but it's just my thoughts...
If you want me to find a suitable push button from RS, give me a shout.
Don't ask questions, just click my rep tick. Now.