Replacement Transistor for Ultrasound Cleaner
I am a former employee. 
It is an NPN High Speed switching transistor (similar to a BUV48A in a TO3 case). Any high speed switching NPN transistor with a rating of at least 600v, 15 amps, with a low hfe should work.
You will also need to check the diode closest to the transistor, because it ran hot and failed in this board version. Use an ultrafast 2 amp. 
Change the 1 ohm resistor (R2) to a 24 ohm 3 watt, and change the 56K resistor closest to the 1uf 400v Capacitor (R4) to a 100K 3 watt. 
You may need to change the turns on the feedback (toroid) transformers, depending on the output waveform. One full cycle of the output on the oscilloscope should be approx. 16.4usec (~60khz). Measured from the Emitter to the Collector. The "on" and "off" should be as close to symmetrical as possible ( 8.2usec: 8.2usec). Approx 350vpp. The turns on the toroid should be 10T and 4T in general.
I suggest using a 2amp GMA pigtail fuse instead of a 1 ohm fuseable resistor.