You can find the schematic of your machine with this link. It is the 4th file (listed as large file) https://mymowerparts.com/pdf/Arctic-Cat-ATV-Repair-and-Service-Manuals/arctic_cat_2000_thru_2009_atv_and_snowmobileSnow_Wiring_Diagrams__NOTE_LARGE_FILE_459mb.pdf There is no schematic of the ECU itself, it is a sealed unit and not serviceable. Since you tried a donor ECU and have different results that makes me suspect your ECU. You probably have a second problem and I would suspect a problem in the wiring harness. Arctic Cats of that era were known for the wires to rub and break. You may need to remove the plastics and inspect the harness from the ECU leading out to the sensors.

Thank you, the fuel pump is working directly, and all the electrical wiring has been tested under load using a 55-watt light bulb. The fuses are all intact, the battery is charged, and the starter turns smoothly.

Going to need a puller to get the bearing out, (it has a grooved lip on it that sticks out, which is ultimately WHY the newer retainer plate will not fit over it) and I used a really big socket that was big enough to sit on the outer race to tap the new bearing into place. You may also want to get the proper front sprocket that goes with this setup, as I had to use a couple 14ga. bushings to shim mine. If you want to save a little $, find a bearing #206 or 6206 from any local auto parts store. same bearing at a significantly lower price than Suzuki. Same with oil seals and o-rings. In the parts diagram, they give you the dimensional sizes of these. Just google to cross reference common part numbers.