2008 honda civic with 175k miles and a cracked engine block. How do you repair this? Do you need a new engine?

Should I put the money in to fix the car or try and sell it?

Cheapest option, find a used engine and fit that. That at least gives you a useful car again, relatively cheaply.

Otherwise, sell it as scrap / parts.

Ianab is right - a used engine would be your cheapest route to getting the car going again. Honda had trouble with the engine blocks used in the Civic - mostly in 2006 but a few (like yours) suffered porosity problems for a few years after that.