A single flash means the ECU is bad but that never happens. Is the ECU chipped?
Don't swap the whole engine, just swap the harness and ECU if they've been tampered with. Just look things over and make sure it has the right distributor, all the trans bolts are in and all the mounts are there. Check if the MAP sensor and TPS plugs were smashed.
Most knuckleheads can swap a motor without causing many permanent headaches. It's the wiring where people make no effort to learn how to do things correctly. "Electricity??!
"