Can a lightning detector's transistors be substituted with BC547 or 2N3904?
Direct replacement for the BC109 would be the BC549 rather than 547 (yes there was a BC107!). And the gain grades (A,B,C) line up too so it's really the BC549C you want to replace the 109C (gain about 400 or higher)

BC107 was aimed at switching, BC109 was aimed at low noise amplification, and the distinction lives on in the BC54x series (and their BC84x surface mount replacements)

2N series probably contains a satisfactory equivalent but I'm not so familiar with American transistor series. Look for gain or hfe of 400 or more.