Well, I wouldn't call them mechanics to begin with.
I would start by returning the second hand injectors (well at least 3), because that is not the cause of your problem. If the injector was the cause for the misfire in cylinder 1, you would only need 1 injector, not 4. If the injector is broken or giving not enough than I suspect a burned valve.
Next I would never return to these 2 guys.

There's a very easy test to find out if a valve is leaking, it's called a leak test which is done with compressed air. You can even tell which valve (intake or exhaust) is leaking because you can hear the air in the manifold.

Follow the wiring from the cooling fan. You will probably come to a sensor in the lower part of the radiator. This sensor should switch on your cooling fans.
Disconnect the connector from the sensor. Switch the ignition on, no need to start the engine. Now connect the 2 wires in the connector, your fans should start, If it's a 3-wire connector you have to search a little which 2 wires, but one is for low fan speed, the other for high fan speed.
Are the fans running now? Then either the switch is broken or there's no flow in your radiator and the switch stays cold - thermostat jammed, radiator blocked or faulty waterpump.
If the fans do not run when the wires are bridged, then check all your fuses.