Roark Junior suffers damage to groin [closed]

The answer to the question very much depends on the damage done.

Reconstructive surgery is very much a thing.

However, there have been a few successful transplantations.

In April 2018, The New York Times reported that surgeons from Johns Hopkins, led by W. P. Andrew Lee, had performed the first penis transplant performed on a combat victim and the third successful penis transplant so far. This transplant also included the scrotum, but not the testicles, for ethical reasons. The patient's identity was not disclosed.[15]

To name one of them.

Sin City is also a fantasy movie set in the early 1900s, so one should expect a bit of Suspension of Disbelief It doesn't matter if it's actually possible or not, what's important is that the surgery is what made the villian. And if Hartigan would have just killed him, instead of 'punishing' him, he would have avoided the torture and deaths of countless women by The Yellow Bastard.