Holy Moly.
What a rat's nest.
All because they didn't use a double-sided board.
I cannot find Any component designators that match that schematic.

Is there an issue with the amplifier?
The resistor in question may be a factory fact.
Please provide a schematic and a pic of the underside of the board.

Not too sure about synths.
 But I did run in to circuits that where designed around the slow slew rate of older 4558 ics.
one in particular was a Tube Works amp that had a 12AX7 blow out that toasted the 4558.
I installed a modern version of the ic & the customer wanted to know what was wrong with the amp.
The distortion was NOT the same.
So, I grabbed the ic from the reverb circuit & swapped them around & wallah. Customer happy!
 Another instance was the original Tube Screamer pedal.
Same thing.
Modern ic is too fast. slew rate wise.
By this time I had accumilated a 'stash' of older 4558s.
Installed one of them. Problem solved.

Are you getting a signal through A112 pin 7 & 1?

Can you supply us with a schematic of your circuit?
https://www.voxac30.org.uk/vox_ac30_circuit_diagrams.html

As long as there is not any dc voltage on the amp connectors, you should be ok.

It will be interesting to see what output transistors are in it.

You may be on your own with that one.
What's wrong with it?

Tried a Factory Reset?
Hold down the Panel button & turn the unit on.
All of the LEDs should begin to blink.
If not, you are SOL.
Have you checked each voltage regulator?

Nice work!

In all probabilty, if the amplifer worked before you 'modded' it and now it does not work, then you did something wrong.I would suggest that you go back and check your work.

If you can 'stuff' the unpopulated reverb circuit a proper reverb pan will cost about $25.00.

"Can I reduce the output power in half by eliminating two of the four output MOSFETs in order to make the power amp like that of the 100W 3510 model"
Simple answer: NO.
The mosfets are handling the power of the amplifier.
ie: The voltage rails.
Not too sure how to go about it but the key is how hard the preamp is driving the power amp.

I would 'cut' the traces coming out of the SA21111 at pin 13 (R OUT) & pin 14 (L OUT).
Solder in your aux input after the cut in the traces.

Did you search the web?
I found this.
Not sure of the particulars.