The airplane is likely an older twentyfive sized airplane and that at a time when twentyfives were not as strong. Based on virtually no info beyond just size and nothing on stuctural density then a forty may not be a safer bet at all. Displacement creep exspecially in these forums often has a devistating effect on nice flying airplanes.

The flexcable single servo aileron setup was common in the past especially for smaller airplanes. It can work just fine. A contemperary model that still uses that setup that comes to mind and still sold by Hobby people is the Modeltech AT-6 twentyfive size Arf designed by Fred Reese and also still kitted by Balsa USA I think it is.

John