Apparently these days the PS2 is completely cracked, and you don't need to do any hardware mods as long as you have the network adapter which lets you connect a hard drive to the PS2. Unless you have the last generation of PS2 slim, which doesn't work.

Basically you format a memory card to put some sort of program on it that lets you play games that you load onto the hard drive. To format the memory card you either burn a special DVD specific to the particulars of your PS2, or you format the hard drive with the software that then formats the memory card. Then you use some PC software to load ISOs onto the hard drive.

Back when I did this many years ago I think it required a gameshark disc and some hardware that let you program a memory card from a computer, so this seems like it's an improvement. I may try this with one of my PS2's with a dead DVD drive to see if I can breathe some new life into it.

There's also apparently some knock-off network modules that let you connect a SATA drive, which could come in handy if you want to use a modern SSD to load games off of.