We have a breakfast cereal in South Africa that is called ProNutro. It had been around for many many years, ever since I can remember. I have been very successfully raising doves and pigeons on ProNutro for over four decades. I do know about the crop milk powder and have thought about trying it if I ever find it available somewhere here, however ProNutro is, I am willing to bet, available in every grocery store in South Africa and it works just fine on rescued baby birds that have at least been fed by their parents for a few days. I grew up eating ProNutro for breakfast, only I mixed it then with milk for myself, whereas with the birds I mix it with a bit of warm water.
Birds that I have rescued that are insectivores is a whole different story!

...and penguins.
[Added edit: here is a BYC link to a little story about two of my insectivore rescues...
Wagtails ]