If you are trying to get around 400fps you could go with a laylax 120 spring. That should put you right around 400fps with .2 bb's. If you are looking at pdi springs I believe a pdi 140 spring will put you around 430 fps with .2bb's. Don't hold me to that though I am not sure with the pdi springs. If you are using heavier weight bb's though your fps will will drop a little.
I would recommend upgrading some of your internals though. You should have a zero trigger or reinforced sears, so you don't run into problems with them breaking. A metal spring guide would be a good investment as well, if you don't have these things already.
No it's all standard at the moment. I am actually trying to downgrade the rifle so hopefully they'll last a little while but as with airsoft over time I'll do bits too it. I just need to get it to 400 from 450 fps
What I did with my rifle was I cut down the factory spring until it was right around 400 fps. Now if you are going to do this, only cut the spring ONE coil at a time or you could screw you're whole spring up. A positive side effect to this is it makes your rifle more silent because the piston head isn't hitting the cylinder head near as hard. And as for the metal trigger, there is a fairly decent one for the L96 on hit guns, I'm not saying its the best, but its better than the factory plastic trigger and not nearly as expensive as a zero trigger.