Replacing a 1200v 8A fast switching diode that is obsolete
Repairing and replacing power elements in power converters is a tricky business. In a professional design all components around power switches are designed/tuned to have least possible dissipation in transistors. The switching is usually designed to avoid "active region" of a switch (where the most transient dissipation occurs), and provide something like shown in green in the picture below.

So the complementary parts of high-power switch (diodes and snubber circuits) should carefully match the original components, reverse recovery charge and time, dynamic impedance, etc. A "better" diode might be not the most efficient one without additional circuit tuning. Otherwise the power transistors would go into "puff" in no time. You probably should consult with On semi/Fairchild about EXACT replacement part.