Probably not worth the effort to even put in a new spring. If the gearbox is plastic it will not be able to hold up to the tension used by a heavier spring.
Tech +1: Save up and get a decent AEG where you are age verified. The one you have now, if it is in working condition, will do you until that time comes.
What you have now will probably last longer if you do not start taking it apart and replacing parts.
The old saying goes, "If it's working... don't %^&# with it!"
Hope this helps....
AK-47, G&G SCAR-L,
TM P90, APS ASR101 M4A1, TSD VSR-10, SIG SAUER P226 X-Five...