What should I do if I need to use part of a previous researcher's work (e.g. an instrument or a questionnaire) who passed away?
Find out who inherited his copyright, and ask them for permission.
If he's long dead, his copyright may have expired and the work entered the public domain, but this period depends on governing law, jurisdiction, and numerous other variables. A century since the death of the author is probably a safe buffer but not infallible (and gets riskier the more well-known the author or work is).
Better to find the copyright notice for the work and follow the thread from there to the current holder.