If you are carefull you can see that IRFP450 is rated at 14A / 500V ,
with a resistance of 0.4 Ohm.
The device you want to replace is STW12NA60, and is rated at 12A /600V,
with a resistance of 0.55 Ohm.
There should be no problem using the IRFP50 instead of STW12NA60,
since both are used for switching purposes, and since the IRFP450
is widely available.