Did you mangae to get the cassette off?
If not, the "opposing chainwhips" process is this: Wrap one around the big sprocket with the handle to the right; you will use this to hold the cassette + freehub against the ratchets. Remove as many sprockets as you can (probably top two, maybe three) by hand; when you get to the first one that's bitten into the freewheel body, wrap the other chainwhip around that one in the opposite direction to the first and use it as a handle to twist the sprocket back out of its bite marks. Repeat until you get to the bottom few sprockets, which will be on alloy spiders that spread the load better.
As for freewheel body replacement, it depends how deeply the sprockets have chewed into the splines.
If a replacement is necessary, it's unlikely you'll be able to get something different. Each wheel manufacturer has their own little variation on the principle, so there is no such thing as a generic interchangeable freewheel body; you're limited to what your wheel/hub manufacturer produces themselves. And if it's made of cheese, tough bickies.
Just ride the bloody thing and enjoy yourself!