I'd be looking at the sensors first. When one side breaks you dont notice as you put two hands in together and a single working side will trigger the machine. When the second one goes, the machine stops.
That said, motor bucket PCBs dont always show signs of burning when they are gone. They usually burn up around one of the aluminium towers on the board if you'll see it.
The problem folk have with these is there is no test data published so nobody knows what to test. It's easy when you have spares as you say: swap one sensor over to see if it's that, then a PCB, then wire the 2nd motor in parallel, etc. Just swap components one by one.
One reason we made most Airblade parts non-returnable was people were buying a bunch of them, seeing what worked, and then returning what they didn't need. Nice for them but a pain for us.
Buying a donor machine is a good idea. It's the fastest way to fix it, and as you say, you'll have spares for if it conks out again.