CYMA sr25 gearbox

So my friends gave me his old sr25 he tried to rebuild the gearbox and fried the mosfet in it. He had no plans to fix it or have someone and let me try as I’m trying to get into the sport. I’ve tried to find a replacement but can’t seem to. Is there any recommendations on where to look or do. I’m not sure I have the skill to rebuild it myself especially if it’s fried.