I am an author doing research.
Let's see if I can be of assistance.

Q: What is involved in small craft pre-flight inspection?

A: Do a google search for "POH Cessna 152" and you will see many listings showing a Pilot's Operating Handbook (POH) for a Cessna 152 (small craft).  In the POH you will see a section that describes a "pre-flight inspection."

Q:When do you need to, and what is involved in filing a flight plan for a small craft?

A: Click this link (Aeronautical Information Manual - AIM) and look at Chapter 5, section 1, Preflight, and you will see information related to filing of flight plans in the U.S..

Q:What things are looked at most closely during a small craft crash investigation?

A: Click on this link (NTSB - Aviation Accidents). In the search box type in Cessna 152, or Cessna 172, etc, and you will see some accident reports prepared by the U.S. NTSB.  This should give you an idea of what is looked at in a small craft accident investigation.

Q:Is there still a 'black box' on a small craft?

A: Assuming your research involves (U.S.) small craft that are not operated for commercial purposes (generally known in the U.S. as FAR Part 91 General Aviation operations), there has never been a requirement (that I'm aware of) for a 'black box.'