How can I repair a.cracked.block on a 2000 chevy cavalier

That's a pretty common engine, the replacement shouldn't be too expensive.

Really not recommended to patch up a block, like Guy says find a replacement.

Depends where it is cracked. Generally it is not suggested to attempt repair to a cracked block, but there are exceptions. Did it crack as a result of the coolant freezing or did it get overheated?

There are shops that can repair a cracked block but it would be cheaper to buy a new engine. There are methods to repair a block to as good as new but it really is only cost effective for collector cars.