You have an 'emergency', not enough fighters at the moment and facing a German Sea invasion. What happens in 1941 is not the 1st emergency. It might very well be the 2ndBut they did not have those engines to spare. They were the there to keep the trainers in the air. The production lines were turned over to Merlin's long before. Put the Kestrels in (needed for training) Masters single seaters then you turn off the 1940 tap of newly trained pilots which was the real RAF log jam at that time. That was the whole reason for the vast investment in the Empire training scheme that paid off handsomely thereafter. Even Miles realised the finite supply of Kestrels and flew the first Mercury powered Master in 1939.
Turns out that Merlin production was enough, whatever raids that aimed at the RR factories only did slight damage, unlike raids on Supermarine and Shorts. Trying to come up with a plan after the bombs land on a factory is too late. Germans managed to hit Supermarine about 2 months (?) after Castle Bromwich came on line which took up some of the slack while the Supermarine factory was dispersed. Germans managed not to hit Hawker and Gloster or at least not badly (?) and Curtiss aircraft were beginning to be unloaded at the docks. (they were not ready to use). There were a lot of moving parts.
But the essence of "emergency fighters" or "cheap" fighters (usually not the same thing) is to use engines that are not first line engines so you are not splitting up your 1st line engine production more than it already is. Likewise using wooden construction so not compete with existing aluminum allocations. Sometimes "cheap" just meant making things to a smaller scale so the planes used less raw materials. By using a smaller, lighter weight structure the designers hoped to use a smaller, lighter weight engine. This never worked out quite as well as hoped.