Diode substitution for FMXG26S
The critical parameters are:    


30 ns 90% recovery time which requires low junction capacitance 
1.5V@10A which implies an Rs of (1.5V-0.7Vth)/10A = 80 mOhms and raises junction capacitance 
Rth(j-c)=4 °C/W  
Ir = 100uA max @ -600V  which implies 600V/100uA= 6MΩ 

Diodes Inc and Cree etc will carry suitable replacements.


Here's a short list that may or may not contain the ideal replacement, then compare thermal and other specs 

https://www.digikey.com/products/en/discrete-semiconductor-products/diodes-rectifiers-single/280

update

closest so far was an ST diode. STTH1202  NO Good 200V
instead  consider this 600V
 The Sanken part is still the best (but even those can fail if caps also degrade)

Sanken                  ST
30ns max. @ 0.5A 25°C   33ns max @ 1A 25°C
600Vrm                  600V
1.5V @ 10A              2.9V @ 12A typ **worse, may need bigger heatsink**
100uA @ 600V 25°C       20uA @ 600V 25°C (better)
20,000uA @ 600V 150°C   400uA @ 600V @ 125°C ( better but tested with lower heat stress)
4°C/W to case           2.4 °C/W to case  ( better )


also search "ultrafast 600V diode  Sanken stock"

All these specs above are tradeoffs so Sanken in Japan were pushing the envelope.

My suggested replacement may still run hot and fail early unless heat rise is checked after unplugging. If burning hot to touch, a bigger heat sink may be needed. A DMM can check if voltage has decayed to a safe level before touching. Don't if unsure.