Then you can do some aerobic exercise properly, but not too intense! For example: jogging, Tai Ji Chuan, mountain climbing (slow) and so on. Remember not to play antagonistic sports (football and basketball)
As for fitness, I have also heard from other friends who have pneumothorax, and the effect is very good. But I don't recommend you to do it, because everyone is different from everyone.
Eating to make yourself fat is the best way to prevent the recurrence of pneumothorax. You should pay attention to your diet, eat 3-4 meals a day, and add more protein or something.
May you never relapse!