This is a classic 2 transistor astable multivibrator. Many other small signal or switching transistors can be used, including 2N4401, PN2222 or 2N2222. The 470 ohm resistors determine the LED brightness.
Lower resistance means higher current, and more light.
The RC time constant of the 39K ohms resistor and the 10uF capacitor determines the on time for each side. With the values shown, the flash rate is about 1 cycle per second.