bread board has parasitic capacitances (pF) which can interfere with some circuits. but in this case they are small enough to be neglected as frequency is low enough and used capacitors are significantly larger. one important thing when building RF circuits is to keep the connections short (the shorter the better). this too is increasingly more important with higher frequency. to get optimal performance, i'd probably pack circuit to no more than one quarter of shown breadboard. as mentioned above, you will want to properly bias emitter of transistor used in modulator (connect lower end of R8 to GND, get rid of C10, connect lower end of C6 to collector of audio amp (lower end of R12).