For the SSP45N20A, I'd suggest using the IRF3415. This amp is likely to have several other problems. You should replace all of the driver transistors (double-row, closest to the outputs). You should also replace or VERY carefully check any transistors that have solder connections that show signs of overheating or have had their plastic bodies turn brown.
The IRF9650 is likely an IRF9640. Use the exact replacement.