I had the same problem back in early september, with the same gun. But, I bought the newer black jg gearbox instead. Here's what I know...
1. Would this fit and work properly inside a plastic receiver? Yes
2. If so, would I need a new motor, or will the stock one be fine? The motor won't work, there's not enough torque. I bought a marui eg1000.
Even though it will work, it makes the mag not sit securely and will fall off. I fixed this by sanding off part of the magazine release.
While taking out the motor, the wirse will have to be cut because it's not long enough for it to be pulled out, and the new one will require you to solder a longer wire length.
Buying a new gun is a better choice.
I was a dumbass and tried to upgrade this gun because I was too cheap and poor. And even though it still performs well, the plastic parts are too weak to withstand any game action. The rails are cheap plastic and they can snap off the screws if you attach too many accessories. (ie, if you attach a hand grip, moving it around will loosen the rails)
I use it to kill random bugs around the house, so it's mostly aesthetics I'm into, hence I drybrushed a metal effect on the spraypainted recievers, and I also made a nifty m203 from some pvc.