So, hunted the net for badass transistors in TO-3 package for a while, here the results:
BUX-48A (NPN, 850V, 15A, 175W)
NTE-386 (NPN , 500V, 20A, 175W)
NTE2319 (NPN , 450V, 15A, 175W, fastest one)
BU508A (NPN , 700V, 8A, 175W)
MJ-10015, MJ-10016 (NPN Darlington, 500V, 50A, 250W)
MJ-10024, MJ-10025 (NPN Darlington, 850V, 20A, 250W)
MJ-10000, MJ-10001 (NPN Darlington, 850V, 20A, 175W)
NTE98 aka MJ-10009 (NPN Darlington, 500V, 20A, 175W)
Found some BUX-48A and MJ-10016 Darlington-transistors on ebay quite cheaply, don't know whether the high current gain of MJ-10016 (hFE between 60-600, compared to hFE of 8 of original MJ13091) causes any trouble in the SMPS, but I guess more current gain is better than too little, because high current gain makes the SMPS switch faster, so less heat in transistor.