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.

You do not 'have' to have a scope.
You can see the waveform with your meter set to read Volts Ac.
In your case the tremelo is stopping.
So if you monitor the transistors you can see it.
I would start with Q10.
Base & or the emitter.

Nice job on that schematic, Galaxiex.
Serious work went into that.
Thanks for all of that!!

"Using the schematics found in these posts"
Can you post the schematic that you are working with?
You may have a resistor problem if you replaced all of the timing capacitors.

My first thought is "polarity".
2nd is current draw.
Probably more constraints but that is what comes to mind.

I have never run in to that specific ic before.
But, if you are in to 'experimenting' I say go for it!

At 15 watts output, being a solid state amplifier, I can't imagine that it would draw more than 1 amp at 110Vac.
At 220 Vac it would be half.

I found 2 Ward Airline SS schematics in my files.
Here they are.

Attached is what I have for the Randall G3 schematics.

From the Web: "Made in the sixties for film editing, this little squawk box amp is all kinds of fuzzy fun. You can play straight into the amp through its rattly tiny speaker, or use the headphone jack to run it into another amp ..."

Sounds like a fun project.
The real issues will be the cabinet & the recharge circuit.

Alesis has a 25 w/ 50 w peak amp for under 200 US.
https://www.sweetwater.com/store/detail/TransActW2--alesis-transactive-wireless-2