That is going to be a tricky conversion... the 1/2A motor usually had it's fuel tank incorporated in it's design, meaning we won't have the luxury of adding much weight for a battery and motor... Still more info is needed to even get close to a suggestion. What is the model's total all up weight. What is the prop size it currently uses and is there room for a slightly larger prop. Wingspan and fuse length may also be some additional info that may help.