Re: Radiator/Coolant Leaks
First of all, take care in not overheating this engine. It will warps the heads and can cause the valve seats to drop sooner or later.
Regarding the radiator, if it lost all coolant, then there must be a considerable leak. The radiator is known to start leaking at the seam between the plastic tanks and the aluminum core. Normally a crack starts there, but this can result in a complete separation of the tank from the core when left like this. This is because the coolant system is pressurized and a crack can grow rapidly until it just gives away and separates the tank from the core. This is quite extream, but can happen when left unattended.
To check for leaks, the engine needs to be warmed up, so that the cooling system is pressurized. Depending on the spot, it can be difficult to trace, but with the leakage you had, it should be not too bad to find.