Hey jludwig
Its a good idea to work on her CV fitness which will increase overall efficiency of muscle function and oxygen transport. The increase in fitness will reduce the effects of fatigue.
Then she should investigate a exercise program to help the endurance of specific muscles that are underused. As these are strengthened they can help yoru mothers posture. Which would hopefully decrease the pain in neck and arms.
I suggest going to see a physio to get appropriate advice regarding starting an exercise program, she can see her GP and get referred to local hospital physio for example, or if she wants to visit a private practice and pay then i suggest you investigate the practice to check someone there has treated polio before.
As for aids and braces - there are numerous postural aid braces out there, some very generic and some specialised. We must see her posture to know which is best - another reason to visit GP/Doctor/Orthotist
All the best to you and your mother - heres hoping she can reduce the amount of painkillers being taken with appropriate input