I have found a few mentioned in this post from Tom_T. Such as replacing the cloth covered fuel lines and relocation of the fuel pump.
I also found an interesting website called Dardiagn which hosts many factory docs as PDF, including:
- 914 Weber Service Manual
- 914 Wiring Diagrams from 1973
- 500+ pages of 1974-89 Porsche 911S, 911SC, Carrera 3.2, 911 Turbo (930) TSBs. Some of which I think apply to the 914-6 engine (e.g. Carrera Chain Tensioner)