G&G GR-25 upgrade

I have an older G&G GR25 that I want to upgrade. I know I have to get a new nozzle and hopup for it. While I have it apart what else should I replace to get this gun up to 500 fps (not including the spring). The gun is still in stock form.

Thanks,

Mike

I would just change the motor, spring, hopup bucking and hope for the best. Meaning that the stock G&G mech box can with stand the pressure that will be transferred to the front of the shell. A G&G SR25 isnt the best option for an SR25 because it still uses a standard M4 shell unlike a A&K SR.

Thanks,

Mike