You will find the part # right next to the transistor on the schematic. IRFZ something or other. You will find out how to connect it by looking at its "datasheet" which is readily accessible on the net. It will show a drawing of its package with its 3 leads, gate, source, and drain. If you look at its schematic representation it will show the gate, source, and drain schematically which will relate to your circuit schematic.
You will need a big heat sink for the transistor. It will be dissipating 8 watts of power. From the datasheet you will find the thermal characteristics, like its Θj-c, the thermal resistance between the transistor's guts and it's case, probably 1 or 2 degrees C/Watt. There are many heatsink calculators on the net.
Be sure to power the 555 with at least 9 volts and better yet 12 volts to make sure it can turn the transistor on hard. The drawing says 3-18, but it can not work at 3 volts which kinda makes me wonder about the whole circuit.