Oh, I'm sure the P-39 will be introduced eventually. Along with the Ki-43, Ki-44, and other Japanese planes that are needed to fill out a Pacific ToD planeset.
It actually wasn't a bad plane, at least not by all accounts. It was a match for the Zero at low altitudes (according to some pilots anyway). The engine power fell off rapidly above 12k, which isn't a problem in the MA. THe main problem, for the MA, is that it is a pre-'45 aircraft, so it would see very little use.
The P-63 Kingcobra, on the other hand, might see a great deal of use. It was about as fast as the El Gay at low alt, had good acceleration, and could turn well. In other words, it would be an El Gay clone, performance-wise, but with a 37mm cannon and 2-4 .50s. It did actually see service with the Soviet Union, I believe 1 got a kill over Manchuria. That is the extent of the service, as far as I know.