Overheating, first, is the coolant leaking, or is the level staying
constant? Also is there undue pressure in the rad?
Is the rad getting hot? If not then you have a sticking thermostat. In fact
that would be the first thing to change as it is the cheapest and easiest.
The other possibility is a head gasket problem, but check the thermostat
first.
Also you might have a blockage in the rad, worth flushing it out if you can.
Easily done with a hose.
The key has a transducer chip in it, and this is read by a sensor in the
ignition switch. For some reason yours is having problems. Do you have a
spare key? If so is that the same?
When you have this fault, does the starter turn , but the engine doesn't
fire, or is everything dead?
If the starter turns then the key code is only controlling the cut-off
solenoid in the fuel pump. It would be possible I suppose to wire in a
manual override for this in the shape of a toggle switch in the cab. You
would need a switch as otherwise you would not be able to stop the engine.