Amla oil is phenomenal as is Coconut oil massaged in everyday. Biotin can help too
You can try hoof and coat supplements like biotin which will help the mane and more.
MTG works well for some people. Other than that, probably just time.
There's no real quick fix, but anything that stimulates hoof growth will help with hair growth too so biotin etc. should help, as well as frequent brushing and rubbing in order to stimulate the hair follicles. There's a few products that claim to speed hair growth but I don't know if they actually work. The quickest thing to do is to sew in hair from another horse when plaiting.