You're most likely not going to want to do this because it's expensive as all fuck..
Find an Angel silent piston head (Lol good luck they've been out of production for a
If you can't find that, Sorbo the cylinder head and use the MASK piston head.
Systema Helical Toque up gears
Marui EG700, then get it comm-lathed to improve the performance and reduce the noise
Get a FET with active breaking
Since you don't have room for something like Dynamat, you can't do that... So throw on a foam-filled silencer that has a decent weight to it (Yes silencers actually do change the noise the gun makes, and it eliminated a majority of the "pop")