Sorry, but if your subject is that far, no crop camera would save you. D7100 is the highest pixel density DSLR currently available and the horizontal resolution is only about 50% more. This would be barely noticeable, especially, with lenses like 70-300 VR that far from being spectacular at the long end.
The only solution of this problem is to get closer. Most likely even the longer lens wouldn't help you either at this distance.
People often overestimate importance of megapixels. The difference between 24 and 10 megapixels is in reality smaller than you think.