the power button is just a momentary switch and can be replaced with any momentary switch.
I use a magnetic reed switch (from an alarm system) to turn on my pc with a fridge magnet.
first a few words about safety
Canned Grounding Rant-
shut down system and remove side panel. with the power cable plugged into the PSU touch a bare unpainted metal area of the case. (my favorite spot is an unpainted screw securing the PSU) once you have grounded yourself you can unplug the computers power cable from the PSU and can touch the system.
if you move your feet, or shuffle in your chair, plug in the cord, reground yourself and unplug again.
end canned rant-
now that you are grounded you can unplug the two power wires on the motherboards front panel, see step 18 for a clear image of a typical front panel connector
http://www.tomshardware.com/forum/261145-31-perform-steps-posting-post-boot-video-problems
the two pins on the motherboard that the power switch connects to are the power pins. short the pins with a conductor, screwdriver, key, anything metal, just touch then with the conductor for a moment, and remove. the computer should start.
if the PC starts by manually shorting the pins the switch needs replacement and the rest of the system should be fine