What you mention is an interesting point and it has been talked about various times. Naturally i got curious and some time ago i specifically tested this and i got an engine seizure.
The trick here is to know what to expect. A damaged radiator doesn't immediately shut down engine cooling, instead it leaks coolant fluid.
The amount of coolant evaporating from a damaged radiator would probably depend on the amount of damage to the radiator and how much you are pushing the engine (higher power settings result in higher coolant temperatures and pressures, which means more leaking for a given amount of damage).
Once your cooling system is emptied, it is then that engine damage occurs.
I tried this in a quick mission some time ago and it works pretty much like that. I was in a 110 and got a perforated radiator at the start of the fight. I kept flying normally for the duration of the fight, chased retreating Hurricanes all the way to Dover and then started on my way back to home base.
I had almost made it across the Channel, when the engine started overheating rapidly and seized.
If my home base was near Calais or any other airfield at the narrow point of the Channel, i would have probably made it just fine. However, i specifically decided to test the radiator DM once i got that kind of damage so i decided to fly the long way back to my designated field just to see what would happen.
I don't know how realistic it is or how complex is the modeling behind it, but overall it seems fine to me after doing the test and i like how the DM is not restricted to instantaneous effects. Radiators seem to be one of those progressive aspects of the DM that you can cautiously ignore for some time depending on the situation, but one which can have very important consequences if you don't plan for it.
In my example case, the prudent thing to do if i wasn't testing specifically with the aim of causing an engine failure, would have been to divert to an alternate field. If i had decided to set it down near Calais, i would have enough coolant for the trip plus a couple of go-arounds/missed approaches.
You could easily test it out yourself, without even having to engage enemy AI and hope they hit your radiators. Just push the engine over the limits for a while to cause some intentional radiator damage. The easiest way to do it would be to go maximum power with the radiators closed at slow speed (that is, a full power climb at a steep angle). The coolant temperature will go way up and the resulting increase in pressure will break your coolant system.
From that point on you can just fly around and see how long it takes for the engine to seize, probably using a bit of time compression too. Cheers
