Looking to fix a 1970 Monte Carlo, what do I need to know about it?

My dad has an old 1970 Monte Carlo in my grandmother's garage, and since I don't have a car, he said I can have it if I put money and effort into fixing it up. I've seen the car, what I know it needs is a new engine, as my dad took out the engine and put it in another car that was sold. It also has a rust spot on the back part of the right side, and needs a new coat of paint.

So what should I know or look into about getting a new engine. Is it possible to put a different engine than what it had, or should I just get a new V8 engine? Also, what can I do about the spot of rust on the body? I've never looked into any of this, and it's all new to me, so sorry if I sound stupid about this. Thank you for any help you can give.