It could be a variety of things, without knowing your troubleshooting skills, or the equipment you have to do the troubleshooting, you are kind of just "throwing parts at it" in hopes it will work.
Trace out the RF, and check your voltages. 10 out of 10 times doing this you will find it. Most common issues are the coupling caps between the TX mixer and predriver / driver / final areas. Also check the output network for shorted / out of tolerance caps. Finally it could be the driver or final.
With 1/2 watt of output, I would say the "regulator" is fine, does it have modulation on TX but just with low power out? If so then the "regulator" is fine. I keep putting the word regulator in quotes because it is not actually a regulator, but in the CB world that is what it is referred to. It is actually a modulator in most high level AM CB radios, but we'll save that topic for another day and another post.