The simple answer is that you'll need to swap the spring out of your AEG to lower the FPS.
FPS field limits are ALWAYS measured with 0.20g BB (not the 0.25's you're using)....UNLESS otherwise stated.
That said...you're better off using 0.25/0.28/0.30's to play with. So chrony with 0.20's...play with heavier ones.
Your AK will come apart much like most others...refer to Mechbox.com or just google "[your model] disassembly"
Fair warning...don't try to fiddle with your mechbox the night before a game...it almost never works out
For 300-350...just stick with a M90 or M100 spring
The speed will depend on your setup/parts...so err on the low side just in case