Question
One koi was speared today in its chest by a Heron. I brought it in the house and have it in a small plastic container with an airstone and I added salt to the water. If alive tomorrow I will put it in a larger container when the water is the same temperature. I read on line putting hydrogen peroxide on the wound followed by Mercurochrome. If the lung is punctured can you still do the above? If so how often should this be done or what else should I do?

Answer
Hopefully, the wound is superficial and no major organs functions are compromised.  You should treat the wound directly with Hydrogen peroxide and Mercurochrome, as you said.  One, maybe two treatments.  The wound will heal itself after that.  It is up to you to provide an absolutely ammonia-free environment while the wound is healing.  The water chemistry will indirectly determine how well the wound heals.  You don't have to worry about a punctured lung.  Even if the fish's gill cover is torn off, it could still survive.  Keep the salt levels extremely high to prevent bacteria and parasites from infesting the wound.

In the future, you might notice the fish having a hard time beating other koi to food.  See if you can get an understanding of how the fish's injury has affected it's life, then make adjustments accordingly.

Best of luck.