You might supplement their calcium, replace the loss of electrolytes~ caused by their high metabolism and resulting diarrhea~ with some ACV in their water, and provide weight bearing exercise like having them walk up and down a ramp in order to get food and water, or place the food on one end of the run and water at the other. The loss of electrolytes is one of the reasons these birds have heart failure in stressed or hot conditions.
These birds do indeed forage, so they can and will if you provide free range opportunities from a young age...this will increase leg and heart muscle strength.
Eventually, if you notice they are struggling to walk, gasping for breath after mild exercise or suffering in the heat, I'd urge you to be merciful to these chickens by putting them down....you are expecting normal development and growth/health from a breed that is not genetically developed to give this to you past a certain age. I hope it goes well but, if it doesn't, please be merciful.