I picked up a couple of those CYMA V2 MP5 gears boxes. I purchased them just for the reinforced gear box. But if you are going to buy these as a "drop-in" that wont work.
To make these work as a standard V2 gearbox you will need to change the nozzle, tappet plate, wire harness and selector plate. I would also consider changing out the cylinder, cylinder head and spring.
I really like the G&P gear boxes but have had very poor luck with the Matrix/AIM gear boxes.