Welcome! A few quick things to check:
The ECM won't power the fuel solenoid/injector unless it sees a crank signal-bad crank sensor = no start.
Verify you've got spark while cranking. If it runs on ether, fuel control is likely the issue.
Check engine/frame/ECM grounds-loose or corroded grounds are common culprits.
The square connector you found is correct-needs the Nissan/Hitachi style adapter, not OBDII.
If spark is good and the crank sensor checks out, the ECM or wiring is next in line.
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