In the world of AEG, we have plastic constructed L85A1 replicas from Academy. The Academy L85A1 is V1 mechbox based and is in fact quite upgradeable both FPS-wise and ROF-wise if you know the tricks.
The L85A1 offers great ROF because it has a different gear ratio (check its spur gear and bevel gear). On the other hand, it is NOT true that the L85A1 lacks internal parts. In fact, most parts inside a V2 mechbox can be installed into the L85A1 V1 mechbox. The L85A1 mechbox can use the V2 gears, piston, cylinder, spring and nozzle without any problem. All that you need to worry about is the motor and the spring guide, but the spring guide is difficult to break (unless you go over 400fps), and the motor is widely available out there (it is just an EG560).
The L85A1 gear ratio is of a high speed one, therefore it is less forgiving for the motor (that’s why your motor will have a tough time pushing the gears when a harder spring is installed). To power a harder spring, you need to switch back to the “normal” gear ratio by replacing the spur gear and the bevel gear.
Keep in mind, the Academy L85A1 mechbox is an almost-exact replica of the TM FAMAS mechbox, meaning you can buy replacement parts (including the motor) that are made by TM.
The G&G L85, on the other hand, is completely different.
