The Q13 protection mod is extremely important as there have been hams who have blown Q13 within hours (or even minutes) of getting their new BITX40v3 working and hooked up to an antenna. The cause is because they transmitted on a nearby antenna with their 100W or higher station rig while the BITX40 was receiving on its antenna.
The symptom of a damaged Q13 is that receive still works fine but there is no transmit. Note that although Q13 is a driver for the transmitter circuit, its base is connected to the antenna during receive.
Q13 can be replaced by a 2N3904 transistor (there is an SMD version if desired). 73, AL VE3RRD
The 2nd harmonic is a little too high for North American standards. Add a 100 pF 100V NP0 capacitor across L7 in the output filter. This will reduce the 2nd harmonic by about 20 dB. If you are planning on increasing the output power above the normal 5 to 7W, then you should use a 200V capacitor.