I would bet it would fly fine with a .25 in it, because the Senior Telemaster will fly with as little as a .46, although I have stuck a Zenoah G20 on mine for giggles.

I would look at the firewall and fuselage doublers up in the front, if it didnt look strong enough I would double it all up with some extra balsa sheet and reinforce the firewall with some triangular stock and maybe some thin ply if needed, then I would stick a .40 on it and go have a ball.