Where can I upgrade my M83?

Hi, I live in Colorado, and I got an m83, super cheap price, I got it because I wanted to have a good airsoft project. So far, I've replaced the motor, the iron sight, fixed it up with a high-cap mag, but I was wondering where I could get a gearbox upgrade or something of that sort. Basically, I'm trying to find a place I can upgrade the gearbox, hopefully with a reasonable price. Would like to raise accuracy and FPS thats all. Motor pretty much increased the ROF. Thanks