These amps can't always be repaired without re-engineering the output drive section to work with different mosfets. The originals are what the amp was designed around (cheap way to design an amp using already discontinued therefore discount priced parts) and no substitute parts work. The originals have been out of production for years and there is no old stock left anywhere. Not really worth repairing either IMO because the newer amps are better designed and more rugged, and equal power.
If the outputs are still good in them and the power supply is the only bad section then it's an easy fix, but beware, even if it looks like the outputs are still good, they likely are not and are probably the reason the power supply is blown. I believe the original mosfets are Fairchild SSP45N20A and if they are, NO the SSP45N20B will not work, neither will IRF3710Z, IRF3415, IRFB42N20D and other popular mosfets used in these types of designs, none will work with the output drive circuitry.
This same design or similar anyway and SSP45N20A mosfet outputs where also used in:
Some old Sony Class D's
MTX 81000D
I know all this because I own one of these POS's... a PR1KD