You can put any of the same characteristics or higher in voltage and amperage ..... greetings.
Good afternoon: The KSC5386 Transistor is 1500 volts / 7 Amps / with damper diode ... In theory you can replace it with any transist that has the same characteristics ... But first of all, check that the source regulates correctly, check components Associated with the horizontal output transistor, inspect the fly-back of serca for evidence, for example that there is no loss of high voltage, (cracks, flashes under the plate where it is welded etc.)