ATtiny External Clock not working
Using the two leg crystal is not possible with the specific device. There is only the option to use an external clock source, so you can use an external oscillator like the following one and apply the clock pulses to the CLKI pin (pin 1). 



Unlike the two leg crystal that needs additional circuitry inside the mcu and two capacitors, this four pin crystal oscillator is a complete oscillator that only needs power to generate the clock.