Plus it controls the wastegate and reduces boost.

According to the wiring diagram both Jetronic and ESC-II units are standalone items. There's just one signal connection from the ESC to the Jetronic, probably a signal for the injection as soon as the engine starts cranking.

Probably you could exchange the ESC-II with a modern mappable ignition/injection system that works without a CPS. Some might also be able to control a fifth injector. Using a the cold start valve instead is not a good idea.