Welcome! I am Dr. Altman, a licensed veterinarian and happy to answer your questions today. First, congratulations on your new adoption! I hope you have many years together :)
Depending on the cause of the original injury (trauma, bacterial, fungal, open) it should grow back on its own but it may take some time. The top of the head doesn't have as much blood supply as other areas of the body and can take months to regrow completely.
Unfortunately if there is damage to the hair follicles from the original injury it might not grow back naturally.
1.) High quality organic dog food with no by products
2.) Fish oil in high concentrations to promote a healthy coat and skin at 1500 mg EPA & DHA daily
3.) Vitamin E topically on the area (might not help but can promote healthy coat as well)
I hope this information is helpful to you but please let me know if I can be of any further assistance. I am on-line another 30 minutes but will return later this evening if your respond and I miss you. You don't want to apply a lot of creams or salves because it can damage the hair follicles and stunt regrowth in that area.
If you have more questions or if I can help in any other way, please do not hesitate to ask! If you would like to accept my answer, please press RATE OUR CONVERSATION (I am not compensated in any other way). Bonuses are always welcome. Thanks! If you have additional questions after you rate the question, you are welcome to request me for additional conversations if I am on-line or by beginning your question "Dr. Altman..." or "Dogdoc4u..." and others will leave the questions for me. Good luck to you both!!!