Could be...considering the corrosion that's prevalent on your motherboard.
Something shorted out and the traces on the motherboard burned right up.
There's no way that I can tell you from here what caused the short.
Have you even checked any fuses?
Another scenario is that, whenever there's something wrong with the coin-in optics board, the MPU shoots out that [21] error code as well.
This is why I suggested that you get a spare set of coin-in optics.
You'd have to utilize schematic drawings along with a multimeter, to find out if the corroded damage on the motherboard has anything to do with the error presented...that motherboard was severely short-circuited...very likely beyond a reasonable price of repair.
I hate to use use the word "kaput".
I'd toss it before it does any more damage to the rest of the machine.
Obtaining a replacement motherboard and coin-in optics is your best option for getting this machine to work again - as quickly as possible.
Without you telling us if you've tried this questionable MPU in one of your other S+'s, I couldn't tell you whether or not if this MPU is any good either.
I cannot possibly know from here, how extensive the damage is inside your machine.
You're going to have to test the coin-in optics board, and at least the MPU in a known good working cabinet to try to verify if those parts are any good.
or
You could stick your good motherboard from another machine into this problem cabinet to see if this machine will work.
No matter what, you will lose some Benjamin's from your wallet. lol