Hello iBringsTheRuckus,
I see the 2SD1398 Datasheet says it is rated for 800V Collector-Emitter Voltage, and 120W.
The BU508AF is rated for 700V Collector-Emitter voltage, and 50W.
I did not find a 800 or higher to offer, but a better option to start with, is 497-7196-5-ND which is the same 700V, 8A, but is rated for 125W. This is a manufacture number of BU508AW. Has the same pin spacing, and pinout.
Please check the datasheet to help verify compatibility.