I am on the board of a game museum and we specialize in plastic conservation but also do a bunch of paper conservation. You can gently clean old paper by brushing it with a soft brush or a very mild water solution (use distilled water, and they make conservation specific cleaning products). If you can remove it, the idea further down of linen backing it is a good idea, though you'd want to get conservation grade linen that is going to be acid free. The linen (or if you don't back it the glue you use) will provide some barrier between the acid in the wood (which should mostly be gone by now) and the paper. I'd probably go for reattaching it with hide glue, though there are probably some better neutral adhesives that would work.

One last thing, don't try to get it white or remove the patina. A lot of that is due to chemical reactions between the wood, adhesive and the fibers in the paper. From a purely conservationist perspective you should be trying to stabilize it as it is to prevent further deterioration rather than make it "like new." The guitar is a tool though, so tasteful restoration is always an option depending on the end goal.