You can buy a replacement spring by purchasing a Version 2 AEG spring set.
The spring you will use from that set should be the same dimensions if not exactly the same. I recall replacing it one time and it was not perfectly 1:1 the same, but it still worked perfectly.
Once you've obtained the spring, you will need to use a diagram or video to guide you through opening the zero trigger gearbox. In my opinion, opening and replacing the parts is quite simple, but closing the trigger box after replacing the spring is rather difficult. Hopefully you can close it back up with as little effort as possible!
It's in my personal top 3 "most annoying and frustrating things to do in airsoft". :lmao: