1973 Dart 318 core plug replacement questions

The core plug on the right/passenger side rear next to the oil filter is leaking. Questions:
1. Can this be replaced with the engine in the car?
2. Should I get a steel or brass replacement?
3. What size is correct?
4. Should I use sealer? If so what kind?
5. Will any auto parts store have this part or will I have to order it somewhere?

Thank you, Tom