Having had a lot of experience with elastomeric o-rings in my career I can say that the best thing to do is replace them. When an o-ring hardens it has been weakened and is no longer a vaible seal. Putting them in a "soup" to swell and soften them, and there are a number of things commercially available that will do that, only serves to make bad matters worse. Your plan IMHO is a recipe for a disasterous failure.
Sorry for the negative answer but I have had too many times seen bad things happen and result in spending thousands as a result of trying to save pennies.