I agree with BigBoss, be cautious of it's linearity.
Note that the current it shows as 30mA is nonsense, the bias supply uses about 15mA and from experience of using 2SC1971 devices, you will be exceeding the 2A collector current maximum to get 10W out of them. Having said that, they are quite robust devices and seem to tolerate excessive current for short periods in SSB PA stages. Possibly the 30mA refers to quiescent current with no RF drive.
The 1S1588 has a Vf of about 0.7V at the ~15mA it passes so it isn't far off the bias point of the transistor but you can use 1N914/4148 as substitutes as long as they are thermally bonded to the heat sink.
Brian.