I think the attenuator power rating is correct for the info that WA3SKN gave but the attenuation value should be greater than 2 dB as that amp was made in either a 5 watt or a 25 watt drive option as I remember.
If that is the amp that I am thinking of then it was made in a 5 watt or a 25 watt drive option and as I remember mirage used the power amp stage/stages in both amps then the attenuator was added in for the 25 watt drive model and it should be at least 7 db at a 20 watt power rating or more for a 2 to 1 resistor safety margin.
The attenuation value required is at least 25/5 = 10log5 = 6.9897 dB ~ or a 7 dB then the following calculations should apply.
Also If the attenuator is a single ceramic part and you can't find the actual part then You can replace the attenuator with 3 series discrete resistors for the Tee pad as follows.
25 watts/5 watts puts the attenuator at ~ 7 dB and for the T pad the two series equivalent resistors should be 19.124 ohms (20 ohms is closest value) and the middle shunt resistors is 55.802 ohms (56 ohms is probably OK here).
I used 20 ohms series and 56 ohms shunt for my following calculations of the input attenuator and a times 2 power rating safety factor minimum for resistor power ratings, hence the 20 watt rating of the attenuator in my calculations especially if that attenuator used is a 3 legged ceramic part then the input resistor would set the maximum power rating of the attenuator used with a 2 to 1 safety factor.
The input Tee pad 20 ohm attenuators first series resistor will see ~.707 amps thru it so it needs the greatest power rating and the first series resistor should dissipate a power of (.707^2) amps X 20 ohms ~ or around 10 watts so I would use 20 watts for the first series attenuator component which sets the max 2 to 1 safety factor rating of the attenuator.
The 56 ohm shunt resistor should see ~ .39137 amps so it needs to dissipate (.391375^2) X 56 around ~ 8.577 watts, use 16 watts or greater for this shunt and the last output series R should dissipate around (.3131 amps squared) X 20 ohms so it should be rated ~ 1.959 watts, I'd use at least 4 watts here.
Additionally; I have the high power version of that amp but mine is for 220 MHz and it's at home but I can double check my attenuator value and make some measurements and get back later next week with what I find.