It is reasonable to think you can get 2 watts or so out of that circuit since it looks like the transistors are slammed alternately in and out of saturation like a class C amplifier is. I know from experience that a 2N3866 transistor can produce at least 2 watts in class C configuration if driven hard enough. The frequency will be somewhat affected by the antenna, since it is coupled directly to the resonator. The stability depends mainly on the Q of the resonator coil which should be fairly okay. It would be fun to simulate it sometime.