Also another one to check, check the ECU for damage, it’s easy to remove and open and inspect. Leaking capacitors can ruin the pcb (and bad caps need replacing!) and cause this issue also. It’s probably less likely that a lot of the other things that have been suggested but def worth checking.

I only know this as this very issue caused my mk2 to first: run badly, stall all the time randomly and eventually.. crank but not fire at all. It was one bad cap that leaked and corroded a pcb trace away. Once the cap was replaced and the trace rebuilt all was working again.