Deere used the 590 in the older 345 tractors. The later 345's had the 611 engine. Then all the GX345's had the 611 engine. I am comparing part numbers on the engine electrical system between the engines on the 345s, and they use all the same parts (pulse coils, ignitor module(black box), ignition coils, flywheel, stator) except the part of the harness that connects to the tractor main harness. The earlier 345 with the 590 doesn't appear to use the time delay module according to the wiring schematic, so that appears to have been added into the system when they changed to the 611 engine, but if you put the complete 611 system on a 590, in theory, it should all work. Your GX345 engine harness is the same part number used on the later 345's, so it should just plug in and work. Something obviously changed when you swapped the engine harnesses. If I read your description correct, you have power to the coils. If true, then the pulse coils either aren't picking up the signal from the flywheel magnet, or there is a wiring mixup going to the ignitor module (which I think you're calling the ECU). Both engines use the same ignitor module and time delay module, so we know there is no conflict there. It's possible the different harness is grounding out something. There is also a possibility that there is an issue on your tractor that you haven't found or didn't know about. I always suspect the circuit board beside the battery because they corrode on the backside and do weird, intermittent stuff as they age. Might want to check that and the 2 fuses on that board as well. Let me know if you still can't get it to fire, and I will upload some wiring diagrams for you. Please give me your GX345 serial number if you need the diagram. Thanks, and good luck!