I picked up a beat-up BMX with fat tires (20x4"). No idea of make or model.

The kid I bought it from not only managed to cross-thread both pedals (somehow I got the pedals off and they are still usable) but also damaged the non-drive side crank threads. It's a 3 piece crank, square taper.

I've tried everything to get the crank arm off, but no avail. Decided to just give up and replace the bottom bracket (which might have been needed anyway).

I've never removed or replaced a bottom bracket, so some help would be appreciated.

I assume this tool will work.

Once I have the bracket out, how do I measure it to find a replacement?

As you can see in the 3rd pic, the bike will need more work after that. The next biggest concern is that damage to the chainstay. It doesn't appear to have weakened the frame, so I'm deciding between filling it with JB Weld and sanding it, or having someone weld it for me.