Can I transplant hair in half a coin size scalp necrosis? How long should I wait post healing before I can transplant? (Photo)

It is best to wait and allow the coin-sized area of necrosis to fully heal before transplanting grafts.

In general you would wait 6 to 12 months to transplant an area that was healing from necrosis. If you transplant too early, the blood supply is not yet established and you risk repeating the situation that caused the necrosis in the first place!  
I understand your eagerness. When a situation like this happens the immediate instinct is to correct it as quickly as possible. However the best results are achieved with waiting the appropriate length of time.
Good luck!

This should be allowed to heal completely on its own.  Then at a time in the future, it can be transplanted to fill it in