When you do find it you will discover that the PMU takes more than a prod at the button to reset it. The PMU is a computer within a computer, and it repays extreme care, electrostatically, when you are near it.
Having taken the bottom off the iMac while it stands on its display face, reconnect the power cable to a switched off and grounded power outlet. Remove the battery and wait 10min. (10min is overkill, but you need to be sure that you have reset the logic board by removing all power for long enough.) Install a new battery, but ground yourself on the RFI cage before you do so. Touch no part of the logic board other than the battery cage during this process. With the cap end of a ball-point pen, or similar, press the PMU switch once, and only once (ie, no bounce) for a second or so. Reassemble the iMac and power up. If you still get no startup, you will have to start on the real bit of diagnosis.
The first part of that will be to confirm that all the internals are present.
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