Mark,
Welcome to the list. Your problem has been dealt with in the past as Ed has  
suggested, but I have a couple of questions. The sole of the R22 is made with  
marine grade plywood. This product is all but indestructible and will handle  
almost anything us owners will throw at it. If you could post a couple of  
pictures with an accurate description of your problem and needs, we might be  
able to help you with an easy fix.
Several owners have tried everything from treated plywood to underlayment,  
but the only wood to use is marine grade plywood. It is made the same as 
regular  plywood, but the glues are designed to withstand constant temperature 
changes  and moisture levels.
Unless your cockpit was submerged in water for a lengthy period, I find it  
hard to believe that the stringers have rotted.
Hope this helps.
 
Rummy