If you are OK with a needle one-handed ( one inside, one outside the LE pocket ), then put it back on as it was, and you won’t be concerned pumping or tethering your kite on a really windy day. If you decide to sew it back in place, first pop out the fill and deflate valves to allow the bladder to be moved well away from where you are sewing. You will be going though several layers of sail-cloth, you may want a pair of thimbles on to help push the needle through. Use a UV-treated heavy-duty polyester thread if you can find some rather than nylon. If you can’t find polyester, just tie the nylon thread down in a few extra spots & knots, and do some lock-stitches, as nylon has lower friction than polyester, and can slip knots and loosen easier.
When you are done and have re-installed the valves, put your hand in to feel around and check there are no twist, inflate slowly and watch for signs of twists in the bladder.