I'm pretty sure any Intel Pentium M will work. There are two types: Banias and Dothans.
https://en.wikipedia.org/wiki/List_of_Intel_Pentium_M_microprocessors
The Banias are earlier (slower) CPU. Dothans are newer, and is able to reach fsb of 533MHz vs the other fsb: 400MHz. The stock Watchguard x550 cpu is Banias. If you want to upgrade to Dothans just put your processor in and change the dip switches. If you have the wrong setting nothing will happen, it wont boot.
I think the Pentium M 770 (2.13GHz) is the best choice for upgrade. Probably find it on ebay for $5. The fastest Pentium M 780 (2.27GHz) is overprice. About 4x to 5x more in price for 100MHz of speed is not worth it.
The motherboard has specify dip switches to move to position for either type. There are two sets of dip switches, one by the north bridge and the other by cpu. Looking at the switch, the settings are listed next to the switch.