You can use the glue in the kite repair kit that comes with most kites. Just clean out the area that you need to glue and glue the valve back on. Make sure you put some pressure on it while it is drying. Workd fine for me.
If you don't have any glue ask a kite mate one of em should have it, or go to the beach and ask around. I would assume many would keep the repair kit in the kite bag or car.
That should be easy to fix.
Regarding silicon sealant?? Not sure and I wouoldn't try it simply because it is a sealant and not a glue, I assume it wouldn't be adhesive enough.