How to modify a buck converter for higher current output?
Your output cap is reverse polarity.
It's a PMOS so it's required state is inverted relative to the output of the IC. If it were an NMOS requires a high-side floating drive instead.
MOSFETs require actively pulling it LO or shorting the gate to source to turn off. Simply disconnecting the drive signal leaves the gate-source capacitance charged thus leaving the MOSFET on.

With the amount of effort you're already going to, just build one from scratch using an appropriate buck converter IC. At 50A, you might find a single-phase converter lacking anyways.
Except that this isn't a power supply. It's a charger. And 50A is probably fast charging which means it's also probably a lithium battery which means if you mess up you're going to burn your house down. Don't mess around with charging lithium batteries with half-assed methods that just act like a power supply and not like an actual battery charger. And especially if fast-charging them.
So don't go with a buck converter IC, go for an actual charger IC instead. Or you can add the appropriate high-side drive to your little circuit above, add on a current sensor, possibly replace the diode with a synchronous NMOS, and have an MCU control them to make a fancy MCU controlled charger. And of course, you'd have to read up on the charging algorithms and very carefully test things under safe conditions and close supervision until you know the code is correct, lest you burn your house down.