Weight related

I'm doing practices quite regularly, though still yielding into thoughts from a past incident quite often. I'm thin and underweight (51kg and 174cm). Muscles are infirm. I can't go to a gym, as it is distantly located from where I live and I also feel it's artificial to do. My chest dimensions are just on the border of compliance for me to get into IOCL. Since I am studying for competitive exams, it's not possible for me to stuff myself with as much as possible to gain weight. That'd be unhealthy, and it didn't work in the past. I eat home food with abundant nutrients. Please help me what I must do in terms of food, extra sadhana, timings and any other aspect.