What is the correct replacement for this mosfet?
I've had a little search around for you and it looks like you've got a H50N03J made by a company called Hi-Sincerity. I've found this datasheet here. If you go down to page 4 in the datasheet it explains what the marking on the chip is very nicely.

You might not be able to get this exact chip anymore but at least this will give you the parameters of the MOSFET you currently have