Where space is so limited, I would focus on food and fire. A knife is handy, but you can usually improvise that. I would bother with minor pain killers, but a serious painkiller maybe. Minor medicinal relief can be obtained from tree bark and stuff. I don't like those magnesium things, but in this situation it might be better than a lighter. Not sure. You would need a striker to go with it, so it might just as well be a small knife I guess. Signal mirror would be handy, given the situation. Lighting birch trees on fire works wonders also. Depends on where you are flying maybe. I wouldn't worry about water purification as a priority. You can improvise a filter, and you can boil water. You can heat up water with a rock but improvising a drinking bowl can be tricky, and its nice to be able to carry water also. Again it depends on where you are flying. Perhaps a container that fits in your pocket.

Small water flask, double as signal mirror.
Fill the flask with honey, later with water.
Pocket knife. Magnesium Fire Starter.

Not sure how much room is left.