Replacing a transistor when I can't find any info on it
Yes. Give us the circuit and we will be able to spec a transistor for you.

AND a photo to go on with (and comment on thos below).

A copper side view of PCB as well would help muchly (for the terminally enthused to start in on).  

If you have a working unit so much the better.
 I you can tell us the supply voltage, power taken and frequency it would help greatly. That would be enough to hazard a good gusstimate. Even voltage would possibly be enough !!!

BUT the more you tell us the better.
 For example, telling us that it was in a metal T03 case on a heatsink about xxx big and was made in 2008 or ... would help.

eg - like this? Yes? :-)



May be used elsewhere in system, too?
Board level parts supplier here says 

Item #   Description     Price
 SS8G-PCA080     Generator Board, Acousti-pulse 6580     $91.91
 SS8G-PCA120     Generator Board, Acousti-pulse 6580     $91.91



and... . Transistor is about certain to be the same given the peep of part number seen. 



Item #   Description     Price  

SS8G-PCA120X2    Generator/Driver Acousti-Pulse/Dual     $165.44



You didn't tell us you knew board part numbers !!!!
 More photos

Here  and
 here and
 even Sonix's own photos are useful