FoodItem is 3 32-bit ints bigger than Item, but only one is accounted for nutrition. What are the others? This is important because FoodItem is constructed manually and therefore anything not set manually will be left uninitialized.
So, uh, what does these patches actually do? Does it set an entity ID somewhere? Because that should probably be in the comment.
This should probably have a comment before it.
Please add a #pragma once.
Add it to v3.0.0.
Added.
Should these constants (79, 70) be put in a macro so they're not repeated?
Maybe call this Gui_blit_renderHearts_original? _injection is a bit unclear IMO. Also, does it not let you do = Gui_blit in this line?
I'd suggest re-ordering this so that Classic HUD is setup first, then Food Overlay. That way, Food Overlay's overwrites can just replace Classic HUD's rather than Classic HUD's having to explicitly check.
Overall, looks good!