assuming that the fork has an IS mount and the current rotor size is 160mm, you will need an IS compatible adapter most likely the +20 version. hope and A2Z sell them but you will be better off taking it down to your LBS to get the correct size for your rotor upgrade size.
bear in mind that the increase in rotor size will definitely void any type of warranty claim you could make on the fork if naything were to go wrong, but i suspect that this is quite an old fork and therefore any such warranty would have now expired. there will also be an increase in weight to the front wheel as a result of a larger rotor.
it can be done but given the size of the rotor and the stopping power it will produce i wouldnt recommend anything bigger than a 180mm rotor on that type of fork.
