iMac G5 Resurrection Help

As the title goes, I'm trying to resurrect my iMac G5 (it was our family's first mac). A few years ago I tried to convert it to a external display, but failed at the project and the G5 sat in pieces for a while (with its original display ruined) until now. I bought a for parts iMac g5 with the same specs, 20" 2.0GHz, mainly for the display and screws and other small bits. I have successfully put everything back together and the first two LEDs on the logic board come up but the third one doesn't. When powered on there is no boot chime and no picture but the fans ramp up to full speed. At first I put in the original hard drive in (no OS, probably has random files on it) but I have removed it now. USB provides no power to the keyboard (mouse lights up though) and the optical drive will not take in any disks. I have tried our original inverter and the other iMac inverter, I've changed the PRAM battery, and I've reset the SMU with no results. The capacitors are still fine, my father replaced every single one years ago. Right now I am completely stumped and any other possible solutions are welcome.

-rNair