There is a product available just for this from Kitefix.com. Peel2fix is a pack that includes 3 dbl sided bladder taped sqaures that allow you to stick your original valve back on without using glues etc. Most good kites shops are starting to get this product instore, and if they havent then they really need too.
We repair lots and lots of old Slingy and Naish valves each week using these kits. They are $17.95 for a pack of 3 (3 x valve repairs.) Anyone can use it no probs.
Also there is a great product for tears in canopy and LE, Using a mesh and glue compound, Get amongst it....
kitefix.com/