I guess you could just get a fledgling "gen2" motor. Brushless is standard for the gen2. Or you could really get any other 1000kv, 350/450-ish watt motor. You'll probably want to change out your prop to something in the 10x5e range with the gen2 motor, That's what's recommended for the gen2 anyway. You may or may not need a prop adapter, depends on whether your stock speed 600 has the same size shaft as whatever brushless motor you buy. Some brushless motors will come with an adapter included. You'll need a motor mount (which some motors will come with also) if you buy an outrunner, and you'll have to get a little creative in mounting the motor.
Hobbypeople carries them, among others. You'll want a 40 amp esc to go with it.