Years ago i built a 2 transistor FM transmitter similiar to this one. after trying several transistors for the end stage, i think i got about 4 watt at around of 22 volt instead of 12 volt only. i used a very big heat sink ( 20 cm x 20 cm square ) for the high frequency output power transistor. I possitioned it at the roof of our 9 floor high apartment with self a made 80cm vertical teleskopic dipole antenna. At the end i could listen my radios antenna about 5km in open area.

Good luck to everyone experiencing in this field.