Let's try and get some info.
I wish you would of given us a little history of your machine.
Did you ever see it working?
A red LED on the MPU is not good.
Check underneath the MPU you slid out earlier to see if there's any pins folded over & touching each other.
Also, never install or remove anything from your machine with the power switch ON.
We need to see a photo of your MPU.
Usually a "green" battery is the Varta battery.
It's not needed for home use.
Those Varta batteries can easily be 20+ years old, and leaking acid all over the circuitry.
A dead one almost certainly means that it's already been leaking acid and slowly destroying your board.
Since you've said that you've re-seated the power supply unit (PSU), let's check how much voltage is going to the backplane board, from your PSU.
Open the door, turn on the power switch.
Look at the very right bottom of the backplane board - on the back wall of the cabinet.
At the very bottom is a white connector plugged into the backplane.
Next to it may be some words etched on the circuit board that say "Power"?
Anyways, that connector is plugged in there vertically.
Set your multimeter to check DC voltage....set if for say 200 volts.
Stick the red probe of your multimeter into the left side of that vertical plug where the red wire is coming out of it.
Touch the black probe of your multimeter onto the metal floor of the cabinet.
Note the voltage.
Now pull the red probe out and stick it into where the orange wire is.
Touch the black probe onto the floor again.
Note that voltage.
Finally, stick the red probe into the top right of that white connector where that single black wire is.
Touch the black probe of your multimeter onto the floor.
Note that voltage reading, turn off the power switch, and report back with your findings.