it is kinda hard to do. yo need to take all the keys off carefully and get to the contact points, it will look like a plastic film with traces on it. look for faded and open traces and use conductive pen to fix the traces. http://www.all-spec.com/products/CW2000.html
open your imac up. trace back from your power switch to where it is on the motherboard. use tweezers to jump the contacts to switch on your power. this is the cheapest way to solve your problem. if your have some soldering skills you can solder a reset switch from a old pc.